Mr Johnson has insisted the post-Brexit transition period will end in December 2020, regardless of whether he has agreed a new trade and security agreement with the EU by that deadline.
But Catherine Barnard, the professor of EU Law at Cambridge University, warned it was “very unlikely” a complex new deal would be agreed by then – and that an extension to the transition must be requested by June 2020.
“That process of requesting an extension, of course, will have to happen within three or four months of us having left,” she told the MPs.
And she warned: “There is a process by which parliament can scrutinise whether a request should be made, but not actually trigger the asking for an extension.”
One Tory Brexiteer on the committee said the process sounded “depressingly complicated” and questioned whether it would be “easier to leave without a deal”.
The controversy lay behind the pleas, by Tony Blair and others, for Labour to stiffen the legal protections against a no deal, rather than agree to the pre-Christmas election now to be held on 12 December.
Suspicions that a no deal at the end of 2020 remains the aim of hardline Tory MPs were first prompted when one said it was their reason for backing Mr Johnson’s deal.
Ms Barnard said the UK was on course to leave the EU on 1 February, if the withdrawal agreement bill is passed following a Conservative victory.
Eleven months “is very short indeed to negotiate an international trade agreement, especially one of the ambition and scope that the political declaration indicates”, she pointed out.
Christopher Chope, a Conservative hard Brexit supporter, suggested: “It would probably be easier to leave without a deal on 31 January, rather than get ourselves into the ‘neverendum’ of trying to get a future political relationship sorted out within such a short space of time.”
Extraordinarily, the political declaration agreed by the UK and EU says the future negotiations “will convene at a high level in June 2020” – the deadline for making the extension decision.
Jack Simson Caird, of the Bingham Centre, an independent research institute, said the negotiating mandate would have to be approved by parliament first.
“It might be that the negotiations haven’t even properly begun in a way that could lead to them being concluded by the end of the transition period,” he warned.
The controversy comes after Matt Hancock, the health secretary, admitted the Conservative general election manifesto may not contain firm proposals to solve the social care crisis.
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