Labour’s shadow cabinet witnessed a top-level bust-up as senior frontbenchers pressed Jeremy Corbyn to give clearer support to a second Brexit referendum.

Shadow chancellor John McDonnell warned that the current stance looked “indecisive” and risked the party being seen as “triangulating” between Leave and Remain. He told the meeting: “We need to make our position clear.”

Deputy leader Tom Watson, who has been pushing Mr Corbyn to give full-throated backing for a Final Say vote, said the party now needed to move “swiftly, decisively and with humility”.

He added it may be “too late” to stop a no-deal Brexit if Labour delayed a change in its stance to its annual conference in September – as Mr Corbyn has suggested.

And Barry Gardiner, who has previously argued strongly in favour of a Brexit deal, said that he now backed a “remain-and-reform” position. In an apparent criticism of Mr Corbyn, he said that recent election results were down to “failure of leadership”. Shadow foreign secretary Emily Thornberry said Labour should be “true to our internationalist values and campaign for remain and reform”.

In a statement to his shadow cabinet, Mr Corbyn made no changes to his position, saying he had stuck “faithfully” to the policy agreed last year, which prioritises a general election over a second referendum. He said he wanted to consult further with MPs and trade unions.

Repeating his statement after last month’s disastrous European elections, he said it was “now right to demand that any deal is put to a public vote”, which could be a general election or second referendum. And he said that any referendum ballot paper should contain “real choices for both Leave and Remain voters”.

Leave campaigners were hopeful that the Labour leader was inching towards a more explicit pro-referendum stance in an imminent speech, after he said he would “set out our views to the public” following consultations.

Ahead of Wednesday's meeting, Mr Corbyn received a letter from 26 Labour MPs mostly from Leave-backing constituencies who urged him to support a deal to take the UK out of the EU by 31 October. They warned that a shift to a pro-Remain stance would be “toxic to bedrock Labour voters”.

The open letter, signed by frontbencher Gloria de Piero and MPs including Stephen Kinnock, Caroline Flint and Lisa Nandy, said that the near-defeat by Nigel Farage’s Brexit Party in the Peterborough by-election was a “stark warning” of the risk to the party.

“The strength of the Brexit Party in Labour heartland areas in the European elections revealed a much more potent threat than either the Liberals or Greens present,” they wrote.

And a briefing note believed to have been drawn up by the leader’s office warned the shadow cabinet: “There is an evident risk that shifting to a more explicitly pro-Remain position would leave us vulnerable in seats we need to hold or win without enough potential seat gains in winnable Remain majority areas.”

However, the People’s Vote campaign released its analysis of recent polling by YouGov, which suggested that 51 per cent of Labour’s 2017 voters deserted it for a pro-Remain party in the European elections, while just 14 per cent voted for the Brexit Party or Ukip.

Former YouGov president Peter Kellner said that failure to appeal more decisively to lost Remain voters would “condemn Labour to its fourth consecutive general election defeat”.

Speaking after the meeting, frontbencher Rachael Maskell, a supporter of the Love Socialism Hate Brexit campaign, said the party urgently needs to present an “energetic and enthusiastic” pro-Remain message.

“Only a radical Labour message can keep us in Europe, and only by opposing Brexit can we be true to our values and set out a radical vision to transform the country,” she said. “Our position must be crystal clear before Boris Johnson walks into No 10.”

Ian Murray MP, a supporter of the People’s Vote campaign, said: “Step by step the Labour leadership are coming closer to the increasingly clear view of the party’s supporters, voters and members, that we must have a People’s Vote on any Brexit outcome and that staying in the European Union must be an option in that Final Say referendum.

“As Tom Watson said earlier this week, Labour’s values are such that when a People’s Vote comes, it is impossible to imagine Labour will be anything other than a party fighting for us to stay in Europe. Some unelected advisors around Jeremy Corbyn may wish to resist that, but the party’s grassroots will not let that happen. The resistance of that increasingly isolated group to letting Labour speak from the heart on this issue is still doing our party electoral damage and must come to an end.”

And Momentum activist Alena Ivanova, of the pro-Remain Another Europe is Possible movement, said: “Labour is a mass movement, and many of the activists recruited to the party by the hope and radicalism of Jeremy Corbyn’s leadership are watching on in disbelief as, once again, we are treated to more fudge and consultation, and no clear movement towards Remain. We need to move our position before the summer break .”

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There was speculation that after a policy shift, Mr Corbyn may give free rein on advocating Brexit to figures such as chair Ian Lavery. He told shadow cabinet that he had been reading a biography of former leader Harold Wilson, who allowed his ministers to campaign on opposite sides in the 1975 EU referendum.

Change UK MP Chris Leslie, who quit Labour in February over Brexit, said: “Labour’s reluctance to argue for remaining in the EU is a historic betrayal and Jeremy Corbyn has now run down the clock with his continued contortions.

“Revoking Article 50 is now the only practical route that allows the British people the time and space to have a genuine Final Say.