The former Tory leader said defying the law was “the very, very last thing you should contemplate” and said he had “misgivings” about Mr Johnson’s tactics.
At the same time a series of Tory big beasts told the Standard they were ready to vote against the Internal Market Bill in its committee stage unless there were clarifications or concessions, including former Cabinet ministers Andrew Mitchell and David Davis.
Former de facto deputy PM Damian Green is expected to abstain tonight at the Second Reading division along with others. Geoffrey Cox, the former attorney general, declared it was “a matter of honour” to obey treaties signed in good faith.
Senior MPs are convinced that the Government is getting ready to announce an 11th-hour concession, though probably not before next week’s line-by-line voting in committee stage when the revolt is likely to grow.
No 10 sources were taking a firm line this morning, refusing to rule out removing the whip from rebels and adding: “All options are on the table.”
In a possible olive branch, the policing minister Kit Malthouse promised that “the problem goes away” if the EU confirms the UK will be able to export foodstuffs to Northern Ireland from the mainland unhindered.
Mr Johnson has asserted that he feared a “blockade” of shipments to the province, which the EU’s Michel Barnier denied yesterday.
He said: “Passing an Act of Parliament and then going on to break an international treaty obligation is the very, very last thing you should contemplate.
“It should be an absolute final resort. So I do have misgivings about what’s being proposed.”
He added: “But I would just make this point. So far what’s happened is the Government has proposed a law that it might pass, or might not pass, or might use, or might not use depending on whether certain circumstances do, or do not appear.
“And, of course, the bigger picture here is that we are in a vital negotiation with the European Union to get a deal and I think we have to keep that context, that big prize, in mind.”
Mr Mitchell, the former international development secretary, said he would back the Bill tonight but might rebel in the committee stage if there was no concession.
He said the EU appeared to have acted “in bad faith” by declining to give Mr Johnson a Canada-style trade deal offered to Mrs May, but he went on: “What I really do not think we can do as law-makers is deliberately vote to break international law, and on a treaty only recently signed by Britain. I do hope the Government will think again.”
Speaking to LBC, he said: “Boris Johnson is all over the place.”