Sir Roger Gale said that if no free trade agreement is secured by the end of the transition to Brexit, just 17 days away on 31 December, Mr Johnson would have “failed the people of the United Kingdom” and his position as PM would no longer be tenable.
In those circumstances, he should stand aside and make say for “somebody more able to pick up the pieces”, said the North Thanet MP.
Gale is the first serving Tory MP to call on the PM to resign if he fails to reach a deal.
In the House of Commons since 1983, Sir Roger is one of the Conservatives’ longest-serving MPs and a former vice-chair of the party.
But he is a long-standing critic of Boris Johnson, who voted Remain in the 2016 EU referendum, backed Jeremy Hunt in last year’s leadership election and has spoken out against hard Brexit. As such, it is unlikely that his stance will be echoed by more than a handful on the Tory benches.
Speaking a day after the PM and European Commission president Ursula von der Leyen put off the latest deal deadline yet again, Sir Roger said: “If Mr Johnson fails to reach an acceptable trade agreement with the EU, the prime minister will also have failed the people of the United Kingdom.
“As a lifelong member of the Conservative Party, I believe that Mr Johnson's position as prime minister would be untenable.
“As an honourable man, he would have to make way for somebody more able to pick up the pieces, to re-unite the whole country and to show the leadership that Great Britain and Northern Ireland deserves.”
Sir Roger later told the BBC that he wanted a free trade agreement that was “acceptable” for the UK, and not “any old deal”.
Failure to deliver this would amount to breaking a promise made to voters at last December’s general election, he said.
And he said: “The Prime Minister promised very clearly to the British people at the election that a deal would be easy to achieve and that the deal would be delivered.
“I believe that deal is in the interest both of the United Kingdom and the European Union. And I want an acceptable deal to be reached.
“If the Prime Minister fails, yet again, to keep a promise. And I believe at that point his position does indeed become untenable.
“As an honourable man, he would have to do as David Cameron did, of course, after the referendum and make way for somebody else.”
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