Labour Party members will have the opportunity to force Jeremy Corbyn to commit fully to a fresh Brexit referendum, after the party bowed to pressure to stage a consultation.

The grassroots survey will be launched next month and conclude by early August, The Independent can reveal – paving the way for the policy shift as a new hardline Conservative prime minister arrives in No 10, supporters believe.

The Labour leader has been resisting calls – led by his deputy Tom Watson – to change his “mealy-mouthed” position on a Final Say vote, by agreeing a ballot of members, or a special conference.

Now the consultation will be staged by a branch of Labour’s National Policy Forum, called the International Policy Commission, and reach a verdict long before an expected clash at the annual conference in late September.

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“Once this is put out to party membership, I think there will be an overwhelming response in favour of a commitment to a second referendum,” said a source close to the policy forum.

“By August, before we go into the conference, it should be clear what our policy is – because it is clear what the membership wants.”

Mike Buckley, director of the Labour for a Public Vote campaign, hailed the move, saying: “This could be the breakthrough we are looking for.”

It was also welcomed by Mr Watson, who told The Independent: “Party members were loud and clear after the EU elections result that they wanted a say on our Brexit policy.

“That needs to happen as soon as possible and certainly before conference, when we will be just a month away from a disastrous no-deal Brexit.”

Clive Lewis, the shadow treasury minister – who has warned Mr Corbyn’s own position is “in peril” over his Brexit fence-sitting – called for the result of the consultation to be binding.

“The overwhelming majority of Labour members want us to energetically back a public vote and remain. We are a radical, internationalist party and we need to start behaving like one,” he said.

It is unclear who has given the go-ahead for the survey, but there is speculation that it has been arranged by the leader’s office, to manufacture a climbdown.

Many believe Mr Corbyn now recognises a shift to pushing for a referendum unequivocally is now inevitable – despite his public caution over being “not at the stage yet” – and believes it is better done on his own terms.

The alternative would be another blazing row dominating the conference, like last year, when it is likely that the pressure will be overwhelming.

There is recognition that the policy of “constructive ambiguity” has run out of road and that Labour must respond to the likelihood of Boris Johnson grabbing power on a vow to carry out a no-deal Brexit on 31 October, if necessary.

Many in Labour believe the internal pressure will shift to demanding the revoking of Article 50 altogether, rather than a referendum, if he sits on the fence much longer.

The International Policy Commission develops policy on foreign affairs, international development and defence, as well as the EU, containing about 20 members, including from the shadow cabinet.

The consultation will set out detailed options, including campaigning for another referendum in all circumstances. “I would certainly expect that question,” the source added.

The result will not be binding – the annual conference decides policy – but would be very difficult for Mr Corbyn to shrug off, even if he wished to.

Labour MPs are becoming more outspoken, with Commons cleaners said to be “mopping up the blood on the carpet” after clashes over Brexit at a meeting with the leader last Monday.

Detailed research, revealed by The Independent, showed Labour stands to lose more than 40 per cent of voters from 2017, overwhelmingly to Remain-backing parties, making a Commons majority impossible.

Notably, the party urges its members to “have you say” through the policy forum, tweeting on Friday: “Labour is a democratic socialist party – we make our policy together.”

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Mr Buckley said Mr Corbyn was showing signs of bowing to the inevitable, saying: “Jeremy has been knocking on doors in Islington and being met by Labour voters saying ‘we are not going to vote for you, we’re voting for the Lib Dems or the Greens’.

“There is nothing that winds up a politician more than coming second in their own seat, as we did in the European elections.”

Behind his public statements, making clear his reluctance to push for a referendum, Mr Corbyn has already shifted significantly, by backing one on “any deal” that might go before the Commons.

However, the pressure is to go further, by giving what one Labour figure called “full throated” support and – crucially – making clear he would campaign to Remain in the EU.