The Labour mayor of London, Sadiq Khan, has issued a dramatic call to his party’s members to dump Jeremy Corbyn, saying he has been a disastrous leader who must share a large part of the blame for Britain’s forthcoming exit from the EU.

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Khan, who secured the biggest win by any Labour politician against the Conservative party in more than a decade when he beat Zac Goldsmith to the mayoralty in May, throws his support behind the challenger Owen Smith, arguing that Labour faces all-but-certain defeat at the next general election without a change at the top.

The brutal intervention by Khan in an article for the Observer will give a huge boost to Smith’s camp, and comes as the party prepares to send out ballot papers this week to more than half a million members who are eligible to vote in the leadership contest.

Khan says that while Corbyn – still the clear favourite to win the contest – is a “principled Labour man” whose ideas have brought hope to many, his year in charge has shown he is not up to the job and is “extremely unlikely” to lead the party back to power. Khan’s message is that the party has a duty to oust Corbyn before it is too late.

“Jeremy has already proved that he is unable to organise an effective team and has failed to win the trust and respect of the British people,” Khan writes, adding: “Jeremy’s personal ratings are the worst of any opposition leader on record and the Labour party is suffering badly as a result. He has lost the confidence of more than 80% of Labour’s MPs in parliament – and I am afraid we simply cannot afford to go on like this.”

But it is his comments on what he regards as Corbyn’s abject failure to rally his party behind a pro-EU message before the referendum that will be seen as the most damaging. Khan, in effect, lays much of the blame for Brexit at Corbyn’s door and that of his advisers.

Facebook Twitter Pinterest Leadership hopefuls Jeremy Corbyn, left, and Owen Smith. Photograph: Christopher Furlong/Getty Images

“Throughout the campaign and aftermath, Jeremy failed to show the leadership we desperately needed. His position on EU membership was never clear – and voters didn’t believe him. A third of Labour voters said they did not know where the party stood on the referendum just a week before polling day.

“And you can’t just blame a ‘hostile media’ and let Jeremy and his team off the hook,” he writes. “I know from my own election – up against a nasty and divisive Tory campaign – that if we are strong and clear enough in our convictions, the message will get through to the public. That’s a test that Jeremy totally failed in the EU referendum. Why would things be different in a general election?”

Smith said he was “hugely honoured” to have Khan’s support. “Sadiq ran a fantastic campaign to win power for Labour in London this year, securing a huge mandate from party members and the electorate,” he said.

“He showed that a vision of hope and optimism can win, if it’s backed up with a credible plan to deliver real meaningful change for people’s lives. Since that election we have seen what a difference Labour can make when we hold power.”

Khan’s decision to go on the offensive represents a sudden change in approach by the mayor. Days earlier, in a BBC radio interview, he refused to be drawn on who he would vote for, saying that he preferred to stay out of current internal party arguments, including the debate over who should be leader. But his aides said he had come under growing pressure from supporters and friends in the party to declare his hand and felt it was his duty to make his position known.

Khan lays heavy emphasis on his fear that another four years of Corbyn as leader will ensure another Tory government, leading to more cuts and damage being done to the country’s industrial heartlands.

In an interview with the Observer last weekend, Corbyn blamed any failure to get his messages on policy across on mutinous and disloyal MPs and others who had never accepted his leadership. “We have done our best to get our message out,” he said. “It hasn’t been helped that prominent people in the Labour party – Labour MPs – have spent the last 10 months actively being unsupportive of our policies that have been generally agreed and supported. Now everyone agrees that anti-austerity is the right line to be taking.”

Corbyn said he was using his second campaign for the leadership in 12 months as a dry run for a general election: “Since the resignations in July and the leadership contest, then clearly the public mind has become focused on the leadership contest rather than the future. I’m trying to turn this leadership campaign into a campaign of how we would run a general election. How we would win back those areas of Britain that have become disillusioned – left-behind Britain.”

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With Corbyn still expected by most MPs to beat Smith and renew his mandate, many senior Labour figures are beginning to focus on how to bring the party together afterwards. Tom Watson, the deputy leader, will back efforts to restore a system of elections to the shadow cabinet, which will be considered by the party’s national executive committee in September as a means to rebuild trust between the leader and MPs.

Explaining his backing for Smith, who is the former shadow work and pensions secretary, Khan says: “On the big issues Owen and I have been on the same side of the argument, including opposing the Iraq war. Owen led and – more importantly – won our fight against the Tories’ unfair cuts to tax credits and disability allowances, which would have hurt the most disadvantaged people in our society.”

He adds: “Simply opposing Tory policies will never be enough to help the people we exist to support. At best, you knock off just the very sharpest edges of the Tory project. Winning elections is how you really make a difference. Only then are you in control – able to shape the agenda and implement Labour policies to create a fairer and more equal society.”

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