Time has already effectively run out on attempts to ratify the agreement by 22 May, they say – despite the prime minister insisting talks with Labour can still deliver a compromise before the deadline.
The verdict puts the Conservatives on course to lose most of their MEPs, polls suggest, as Leave voters protest at the failure to deliver Brexit, a disastrous result that would trigger huge pressure on Ms May to resign.
The staging of the elections will also be a personal humiliation for the prime minister, who repeatedly told MPs they should not take place, three years after the Brexit referendum.
More talks with Labour are planned, as No 10 claims the complex (Withdrawal Agreement) Bill – required to ratify EU withdrawal – can clear parliament before voters go to the polls on 23 May.
But two respected think tanks have told The Independent the timetable is a fantasy, with one suggesting it will take “several months” to approve the legislation, which could involve up to 100 votes.
Approval of Maastricht took 41 days in 1993, while Lisbon required 25 days in 2008 – but there are only 17 sitting days planned before 22 May.
Joe Owen, programme director at the Institute for Government, said: “It looks highly unlikely the government will be able to pass the (Withdrawal Agreement) Bill by the end of May, given the political challenge facing the prime minister.”
Dr Alan Wager, a researcher for the UK in a Changing Europe think tank, said: “It’s likely to take several months to pass the bill, given how long comparable legislation has taken to pass historically.
“Looking at the window before the European elections – which is three weeks maximum [after the Easter recess] – there is no way MPs are going to agree to that, no chance of that whatsoever.”
Dr Wager predicted a flashpoint prompted by Remain MPs over a Final Say referendum, as well as Brexiteer anger over the deal, saying: “The government is going to be squeezed from both sides.”
Addressing MPs last week, after a late-night Brussels summit agreed a further Brexit delay until 31 October, the prime minister insisted a rapid passage of the deal was still possible.
“Let us then resolve to find a way through this impasse so that we can leave the European Union with a deal as soon as possible, so that we can avoid having to hold those European parliamentary elections,” she said.
Jeremy Hunt, the foreign secretary, argued the predicted disaster for the Tories of asking voters to elect MEPs could still be averted.
“I think the absolute priority for Theresa May is to get Brexit over the line before 23 May, so that we don’t end up fighting the European parliament elections,” he said.
But Dr Wager pointed out the government had lost a staggering one-third of Brexit votes since last summer, with a further 60-100 likely when the bill arrives.
And Mr Owen warned the bill could “end up so badly amended” that it contradicted the deal itself, leaving the European parliament unable to ratify it.
Two polls have put the Tories on 16 per cent and 17 per cent for the May elections – far adrift of Labour on 24 and 29 per cent respectively.
The party has never sunk so low since the 1832 Great Reform Act expanded the franchise, prompting Mr Hunt to admit the prime minister could be forced out of No 10 by such a result.
A government sourced admitted to The Independent that Labour will be tempted to allow the talks to run into the sand because of the predicted heavy losses for the Conservatives.
Ministers have floated bringing forward the bill before a further meaningful vote to allow agreed changes to be legally embedded, but this tactic also brings huge risk.
If the legislation is voted down before the stage of debating amendments, Ms May would be forced to prorogue parliament and start a new session, threatening the fracturing alliance with the Democratic Unionist Party.
The cross-party talks remain on the brink of collapse, having failed to resume this week between the politicians because of a lack of progress made by officials.
“The government doesn’t appear to be shifting the red lines because they’ve got a big pressure in the Tory party that actually wants to turn this country into a deregulated low-tax society which will do a deal with Trump,” he said, on a visit to the northwest of England.
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