The Transport Salaried Staffs' Association (TSSA) trade union has commissioned confidential and detailed polling for the Jeremy Corbyn-supporting Momentum group that shows "Labour will get a lower share of the vote in every seat in the country if it has a pro-Brexit policy than if it has an anti-Brexit position".
A briefing paper based on the polling has been shared with leading members of the shadow cabinet, including John McDonnell, to increase pressure on Labour's leader Corbyn to come out in favour of a referendum.
The most powerful conclusion of the research is: "There can be no disguising the sense of disappointment and disillusionment with Labour if it fails to oppose Brexit and there is every indication that it will be far more damaging to the party's electoral fortunes than the Iraq war.
"Labour would especially lose the support of people below the age of 35, which could make this issue comparable to to impact the tuition fees and involvement in coalition had on Lib Dem support."
The TSSA paper also says that the prospects for a new centrist party, which could massively undermine Labour's long term prospects, would be significantly enhanced "if Labour does not do more to oppose Brexit".
The polling shows "17% of Labour's 2017 vote would be 'very likely' to support a new centrist party committed to opposing or overturning Brexit. Another 27% say they would be 'fairly likely' to support a new centrist party".
The paper, marked "strictly confidential" and which is described as "helping Momentum and ourselves prepare in case of a snap general election" has been obtained by me for the Peston show and independently by my colleague Anushka Asthana (in an odd coincidence).
It is based on polling carried out by YouGov during mid-December and early January and subjected to an MRP analysis by FocalData for Labour Friends of Hope not Hate on behalf of the TSSA.
The paper says that Labour would lose a snap election - which some would say makes it odd that Corbyn says he prefers an election to a referendum - but that its performance would be much worse if it supports Brexit in an election.
Perhaps the TSSA's most striking finding is that "if Labour supports implementation of Brexit it would lose a further 45 seats" but would only lose "a further 11 seats" if it opposes leaving the EU.
Of its so-called target seats, Labour would not win any from failing to oppose Brexit, but could win five if it campaigns against Brexit.
The best electoral opportunity for Labour would be provided by a general election in which the Tories campaigned on a so-called 'soft' Brexit, or one where the UK continued to follow EU rules to a great extent, and where Labour opposed Brexit. On that scenario Labour would have 38% of the vote versus 33% for the Tories, and Labour would actually win.
If that analysis is shared by the Tories, it may be a reason why Theresa May appears to be tacking more towards the harder Brexit favoured by the Brexiter purists of the European Research Group in her party.
That analysis is also an explicit challenge to the Unite trade union, led by Len McCluskey, and to Corbyn's closest aides, Seumas Milne and Karie Murphy, who vehemently oppose a referendum and would prefer to push the Tories towards a softer customs-union based Brexit that Labour MPs could support.
Within the trade union movement, there is growing concern that McCluskey and Unite are being too friendly and constructive with the government, to secure a softer Brexit and a commitment from Theresa May to protect workers rights and therefore build a cross-party alliance in favour of Brexit.
"For many of us it sticks in the craw that Len seems to be prepared to prop up the government that gave us austerity just because he hates the idea of a People's Vote" said one senior trade unionist.