Jeremy Corbyn was elected as Leader of the Opposition in September 2015, following the resignation of Ed Miliband after Labour's defeat at the 2015 general election. Disillusioned by a lack of a left-wing voice in the 2015 Labour Party leadership contest, Corbyn stood on an anti-austerity platform. Of the candidates who stood, Corbyn received the fewest parliamentary nominations. Many who nominated him said they had done so not to support his candidacy, but to widen the debate by including a socialist voice. However, Corbyn soon became the frontrunner and was elected with a landslide of 59%.

Corbyn appointed John McDonnell as Shadow Chancellor and promoted several female backbenchers, forming a Shadow Cabinet that for the first time had more women than men. Under Corbyn's leadership, Labour shifted to the left from the centre-ground. In November 2015, he voted against British military involvement in the Syrian civil war. He also opposed the renewal of the Trident nuclear weapons system and apologised for the Tony Blair ministry taking the UK into the Iraq War. In spite of his victory, Corbyn enjoyed little support from Labour MPs, although his support remained strong amongst Labour Party members. In 2016, Labour were widely criticised in the media for their apparent poor performance in local elections; despite this, they had won 34.1% of the PCC vote against the Conservatives' 30%, they lost 18 councillors to the Conservatives' loss of 48, Labour government control was retained in Wales, and Labour achieved a clean sweep of the mayoralties of London, Bristol, Liverpool, and Salford. Following the European Union membership referendum, in which Britain voted to leave the European Union, party opponents accused Corbyn of running a lukewarm campaign for the Britain Stronger in Europe campaign; one major opponent, Angela Eagle, cited his lack of devotion to the Remain cause while simultaneously observing the energetic and itinerant means by which his campaign was conducted. Several resigned from the Shadow Cabinet and Corbyn lost a motion of no confidence by 197–40. Eagle and Owen Smith launched a formal challenge, although Eagle later withdrew and endorsed Smith. Corbyn was re-elected with a marginally higher majority of 61%, the largest leadership election majority of any Labour leader in history.

Although Labour suffered poor results at the local elections in May 2017, at the snap 2017 general election the party secured 40% of the vote with the biggest increase in Labour vote share for 72 years, and forced the Conservatives to form a minority government with Corbyn remaining Labour leader. In the 2018 local elections, Labour increased its share of the vote. In the 2019 local elections, Labour's seat total dropped by 84. In the 2019 European Parliament election, Labour came third behind the Brexit Party and the Liberal Democrats. In the 2019 general election, Labour's vote share dropped to 32%, winning the lowest number of seats since 1935. The result led to Corbyn's announcement that he would stand down as Labour leader. Some reasons for the defeat included concerns about Corbyn's leadership, the party's "ambiguous" position on Brexit, and concerns that the commitments in the left-wing manifesto were "undeliverable".

Corbyn remained Labour leader for four months while the leadership election to replace him took place. His resignation as Labour leader formally took effect in April 2020 following the election of Keir Starmer. During his tenure as leader, Corbyn came under criticism in relation to antisemitism within the Labour Party. Corbyn has condemned antisemitism and apologised for its presence within the party, while his leadership oversaw changes to strengthen party disciplinary procedures regarding hate speech and racism as recommended by the 2016 Chakrabarti Inquiry. An internal 2020 report and the subsequent 2022 Forde Report noted that Corbyn's team inherited a dysfunctional disciplinary system which eventually improved under General Secretary Jennie Formby, and stated that antisemitism was used as a factional weapon by both opponents and supporters of Corbyn within the party. A 2020 Equality and Human Rights Commission inquiry into the matter found the party under his leadership was responsible for unlawful acts of discrimination and harassment.

Leadership bid

Following the Labour Party's defeat at the general election on 7 May 2015, Ed Miliband resigned as its party leader, triggering a leadership election. Corbyn decided to stand as a candidate, having been disillusioned by the lack of a left-wing voice, and said to his local newspaper, The Islington Tribune, that he would have a "clear anti-austerity platform". He also said he would vote to scrap the Trident nuclear weapons system and would "seek to withdraw from Nato". He suggested that Britain should establish a national investment bank to boost house-building and improve economic growth and lift wages in areas that had less investment in infrastructure. He would also aim to eliminate the current budget deficit over time and restore the 50p top rate of income tax. He added: "This decision is in response to an overwhelming call by Labour Party members who want to see a broader range of candidates and a thorough debate about the future of the party. I am standing to give Labour Party members a voice in this debate". He indicated that, if he were elected, policies that he put forward would need to be approved by party members before being adopted and that he wanted to "implement the democratic will of our party". The other candidates were Shadow Home Secretary Yvette Cooper, Shadow Health Secretary Andy Burnham and Shadow Care Minister Liz Kendall. Several who nominated Corbyn later said they had ensured he had enough votes to stand, more to widen the political debate within the party than because of a desire or expectation that he would win.

At the Second Reading of the Welfare Reform and Work Bill in July 2015, Corbyn joined 47 Labour MPs to oppose the Bill, describing it as "rotten and indefensible", whilst the other three leadership candidates abstained under direction from interim leader Harriet Harman. In August 2015, he called on Iain Duncan Smith to resign as Secretary of State for Work and Pensions after it was reported that thousands of disabled people had died after being found fit to work by Work Capability Assessments (instituted in 2008) between 2011 and 2014, although this was challenged by the government and by FullFact who said that the figure included those who had died and therefore their claim had ended, rather than being found fit for work.

Corbyn rapidly became the frontrunner among the candidates and was perceived to benefit from a large influx of new members. Hundreds of supporters turned out to hear him speak at the hustings across the nation and their enthusiastic reception and support for him was dubbed "Corbynmania" by the press. Membership numbers continued to climb after the start of his leadership. In addition, following a rule change under Miliband, members of the public who supported Labour's aims and values could join the party as "registered supporters" for £3 and be entitled to vote in the election."How is Labour vetting new members?" , bbc.co.uk; retrieved 20 September 2015. There was speculation that the rule change would lead to Corbyn being elected by registered supporters without majority support from ordinary members. He was elected party leader in a landslide victory on 12 September 2015 with 59.5% of first-preference votes in the first round of voting. He would have won in the first round with 51% of votes, even without "£3 registered supporters", having gained the support of 49.6% of full members and 57.6% of affiliated supporters. His 40.5% majority was a larger proportional majority than that attained by Tony Blair in 1994. His margin of victory was said to be "the largest mandate ever won by a party leader".

An internal Labour Party report, entitled The work of the Labour Party's Governance and Legal Unit in relation to antisemitism, 2014–2019, was leaked to the media in April 2020. The report stated that during the 2015 and 2016 leadership contests, staff members at Labour party headquarters looked for ways to exclude from voting members who they believed would vote for Corbyn. The staff members referred to this activity as "trot busting", "bashing trots" and "trot spotting".

Corbynmania

Corbyn was initially viewed as a token candidate for the left wing of the party and not expected to win. However, many new, young party members, who had joined after the membership fee had been reduced to £3, were attracted by what they saw as Corbyn's authentic, informal style and radical policies. Hundreds of supporters turned out to hear him speak at the hustings across the nation and their enthusiastic reception and support for him was dubbed "Corbynmania" by the press.

Jonathan Dean characterised Corbynmania as a political fandom, comparable with the enthusiastic followings of popular media stars and other modern politicians such as Bernie Sanders and Justin Trudeau. Specific features included use of the #jezwecan hashtag, attendance at rallies and the posting of pictures such as selfies on social media. Artistic, merchandising and other activity consolidated and spread this fannish enthusiasm. This included a "Jeremy Corbyn for Prime Minister" (JC4PM) tour by celebrities such as Charlotte Church, Jeremy Hardy and Maxine Peake; a Corbyn superhero comic book; mash-ups and videos. Many of Corbyn's supporters felt he possessed personal qualities such as earnestness and modesty leading them to develop a sense of emotional attachment to him as individual. These were seen as cultish by critics such as Margaret Beckett who said in 2016 that the Labour Party had been turned into the "Jeremy Corbyn Fan Club".

A chant of "Oh, Jeremy Corbyn" was adopted as an anthem or chorus by his supporters. Sung in the style of a football chant to the tune of a riff from "Seven Nation Army" by The White Stripes, it attracted special attention at the Glastonbury Festival 2017, where Corbyn appeared and spoke to the crowds. Labour's weaker-than-expected performance in the 2018 local elections led to suggestions that Corbynmania had peaked.

First term as Leader of the Opposition (2015–2017)

After being elected leader, Corbyn became Leader of the Official Opposition and shortly thereafter his appointment to the Privy Council was announced. In Corbyn's first Prime Minister's Questions session as leader, he broke with the traditional format by asking the Prime Minister six questions he had received from members of the public, the result of his invitation to Labour Party members to send suggestions, for which he received around 40,000 emails. Corbyn stressed his desire to reduce the "theatrical" nature of the House of Commons, and his début was described in a Guardian editorial as "a good start" and a "long overdue" change to the tone of PMQs. He delivered his first Labour Party Conference address as leader on 29 September 2015. Party membership nearly doubled between the May 2015 election and October 2015, attributed largely to the election as leader of Corbyn.

In September 2015 an unnamed senior serving general in the British Army stated that a mutiny by the Army could occur if a future Corbyn government moved to scrap Trident, pull out of Nato or reduce the size of the armed forces. The general said "the Army just wouldn't stand for it. The general staff would not allow a prime minister to jeopardise the security of this country and I think people would use whatever means possible, fair or foul to prevent that. You can't put a maverick in charge of a country's security".