However, the party is still leaving open the option of seeking to negotiate a better deal if it wins a snap general election – and backing that agreement in a public vote.
Announcing the end of Labour’s consultation on his policy, Mr Corbyn urged Boris Johnson – if he wins the Tory leadership race – to have “the confidence to put their deal, or no deal, back to the people”.
“In those circumstances, I want to make it clear that Labour would campaign for Remain against either no deal or a Tory deal that does not protect the economy and jobs,” he said.
The commitment comes in a letter Mr Corbyn has written to every Labour member ahead of a shadow cabinet meeting to agree a “settled Brexit position” today.
However, he is certain to be challenged at that meeting to go further by making Labour committed to supporting Remain in all circumstances, in any referendum.
On Monday, the big trade unions endorsed Mr Corbyn’s position of seeking to strike an acceptable Brexit deal, if he wins power – prompting criticism that his policy is still a “fudge”.
“Our members and supporters are clear that any kind of Brexit gives us less than we have and Labour should not support it,” Mr Watson said.
In his letter, Mr Corbyn made clear that his priority remained securing a general election – something becoming more likely if MPs block Mr Johnsons’ threat of a no-deal Brexit on Halloween.
“We need a general election. After nine years of austerity, too many people in this country cannot find decent secure well-paid work, and have to rely on public services that have been severely cut back,” he wrote.
“Our country is ravaged by inequality and rising poverty, huge regional imbalances of investment, and the government is failing to tackle the climate emergency facing us all.”
The letter made no mention of what Labour’s Brexit position would be going into any election.
It is understood that Labour's manifesto position will be decided through its ordinary process once an election has been called, rather than in advance.
Phil Wilson, a Labour supporter of the People’s Vote campaign, welcomed the announcement, but said Mr Corbyn must put himself at the “forefront” of the campaign.
“One email to Labour members and a press release do not make a campaign,” he said. “Our voters and our members will not tolerate further confusion or foot-dragging.”
And Tom Brake, the Liberal Democrats Brexit spokesman, said: “Labour are still a party of Brexit.
“It is clear it is still Jeremy Corbyn’s intention to negotiate a damaging Brexit deal if he gets the keys to number ten. Labour must start being transparent with the British people.”
The Labour leader has been under pressure to clarify the party's ambiguous stance on leaving the EU, with concern from pro-Remain quarters that the confusion has pushed voters towards the Liberal Democrats.
In the European Parliament elections in May, Labour came third with just 14 per cent, trailing behind the Lib Dems (20 per cent) and the Brexit Party (29 per cent).
Mr Corbyn has appeared increasingly isolated in the party over Brexit amid reports of splits between his Eurosceptic aides and members of his own shadow cabinet.
John McDonnell, the shadow chancellor, has said that Labour's vague Brexit stance has “not worked” and the party must move towards a Remain position “sooner rather than later”.
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