Union boss Len McCluskey has urged Jeremy Corbyn not to be panicked by the surge in support for the Lib Dems and warned that turning Labour into a pro-Remain party would be “electorally suicidal”.

Blogging for HuffPost UK, the Unite general secretary said a second referendum would “pump more poison” into British politics and would be a distraction from tackling Tory austerity.

McCluskey said that if Labour listened to “Remain zealots”in the wake of the ‘pointless’ European elections, it would lose dozens of Northern and Midlands seats it has held since the Second World War.

Trying to outflank the Lib Dems and Greens – both at record highs in one new opinion poll – would also jeopardise key marginals needed to get Corbyn into No.10, he added.

In a surprise move, McCluskey said that May’s recent attempt to reach out to Labour on a customs union offered a way forward if a new Tory leader resubmitted the plan and if Opposition MPs were allowed a free vote.

However, in a hint that opens the door to Corbyn ordering a fresh review of his Brexit policy, McCluskey admitted that “consulting our membership is essential”, as long as trade unions as well as members were included in the process.

Labour came third in the European elections behind the Brexit Party and the Lib Dems, prompting the Labour leader to declare this week that “the only way out” of the Parliamentary deadlock was either a general election or a fresh public vote.

Unite, which is Labour’s biggest donor and gave it nearly £540,000 in the first quarter of this year, is the union whose leadership is most sceptical about a second referendum, although the CWU postal union has made similar warnings.

McCluskey, who recently accused deputy leader Tom Watson of plotting to oust Corbyn in the wake of the European election results, said a repeat of the narrow 2016 referendum result would not resolve the issue.

“A further referendum will only pump more venom into the body politic. So then we are left with simply cancelling Brexit – revoking Article 50.  For Labour to embrace such a position, as some seem now to be inching towards, would be not just electorally suicidal, it would represent a profound rupture in our movement’s democratic traditions,” he writes.

“There is no way round it – leading the charge for Remain and relying on returning Lib Dem or Green voters, rather than continuing to respect the referendum result, will see Labour losing dozens of constituencies it has held since World War Two and longer, and put key marginals we must win way out of reach.

“Perhaps there would be some corresponding gains in “remain” areas, but anyone thinking that becoming partisans in a ‘culture war’ rather than uniting for social justice is the route to a Labour majority is deluding themselves.

“And there is absolutely no route to a Labour victory as a Remain party, even with transform, reform or whatever tagged on the end – unicorn projects given the way European politics is developing.”

The Unite boss said that Corbyn was right to want to unite the country and focus on a vote of no-confidence in the new Tory leader and a general election.

“Panic is a poor political counsellor.  The bad results for Labour in the elections to the European parliament – elections no-one anticipated,  nor wanted  – should of course be the occasion for a review of where we are, but not for the abandonment of the only plan that could lead to a Labour victory,” he said.

“I have heard accounts of constituency meetings from Liverpool to North London – strong Remain areas – where the members are clear that if given a choice between a Labour government and staying in the EU, they would go for the former every time.”

Corbyn is under intense pressure from shadow Cabinet figures like Emily Thornberry to convene an emergency meeting of the party’s ruling National Executive Committee (NEC) to come up with a clearer commitment to a second referendum in which Labour would campaign to Remain.

McCluskey said that the 500,000-strong Labour membership, and unions, should be now consulted as part of a ‘review’ in the wake the MEP election results.

“Consulting our membership is essential; they are the lifeblood of our party.  But that also must embrace the trade union affiliates,” he said.  

But he also suggested that the Brexit compromise that led to May’s hasty resignation could offer the basis for a cross-party deal in the Commons.

“This may not be a universal view in the labour movement, but I thought the final – and hastily withdrawn – deal proposed by Theresa May could have provided a basis for moving forward.

“If a new Tory leader builds on it – a big ‘if’ I admit – such a deal could come very close to meeting the criteria for agreement that Corbyn himself set out in his conference address last year.

“Of course, many Tory backbenchers wouldn’t like it, but that could be the moment for statesmanship from Labour.  I know that Jeremy Corbyn, as a man of principle, can do that.  And the time for a free vote for Labour MPs, given their strongly held legitimate different views, has surely arrived.”

Amid continuing claims that Labour’s position remains unclear on a referendum, Corbyn this week disappointed ‘People’s Vote’ campaigners by suggesting a fresh public vote would only happen if Parliament first approved a Brexit deal.

“We don’t back a re-run of 2016, that happened, that’s gone. What I do say is if parliament comes to an agreement and it’s reasonable, then there should be a public vote on it, but that is some way off,” he said in Dublin on Wednesday.