The prime minister is trying to unpick parts of the withdrawal agreement (WA) he negotiated last year to give UK ministers key powers to decide when EU rules apply to Northern Ireland.
Northern Ireland secretary Brandon Lewis was forced to admit the proposals “break international law”, triggering a backlash from Johnson’s predecessor Theresa May and Tory chairs of three Commons select committees.
May said the plans would leave the UK less “trusted” around the world while foreign affairs committee chair Tom Tugendhat said the country’s “entire economy” depends on UK adherence to international law.
The PM was also rocked by the resignations of civil servants in key government legal positions.
The head of the government’s legal department Sir Jonathan Jones quit, reportedly over the Brexit plan which will come to the Commons in the Internal Market Bill on Wednesday.
He was followed by Rowena Collins Rice, director general at the Attorney General’s Office, although sources stressed that her departure was long planned and she is set to take up a new public role on Friday.
There were also reports that Lord Keene, advocate general for Scotland, was also on resignation watch in Whitehall.
The PM meanwhile was facing pressure on his other flank as the Tory hardline Brexiteer European Research Group (ERG) warned that “Brexit is not done” unless the PM follows through on the controversial plans.
That was despite Johnson renegotiating the Brexit deal put together by his predecessor May to appease the ERG.
And it was May who led the starkest criticism on Tuesday.
“The United Kingdom government signed the withdrawal agreement with the Northern Ireland protocol,” she told the Commons.
“This parliament voted that withdrawal agreement into UK legislation.
“The government is now changing the operation of that agreement.
“Given that, how can the government reassure future international partners that the UK could be trusted to abide by the legal obligations of the agreements it signs?”
Tugendhat echoed her concerns, telling a British Foreign Policy Group Event: “Traditionally, the UK has been the chief exporter of the rule of law, we have been the single pillar of dependability in international negotiations that has allowed others to prosper and indeed allowed us to prosper.
“On it we have built the goods and services that have enriched our people... from finance to accountancy to pharmaceutical standards to everything else.
“It’s not just about the law, our entire economy is based on the perception that people have of the UK’s adherence to the rule of law.”
Defence committee chair Tobias Ellwood backed the pair and warned that breaking the law would undermine the UK’s attempts to hold increasingly aggressive countries like China, Russia and Iran to account.
Former minister Geroge Freeman meanwhile warned the government may once again be overruled by the Supreme Court, which struck down its attempt to prorogue parliament last year to force through Brexit.
Northern Ireland secretary Simon Hoare said there was still a lack of certainty in the province over what happens after Brexit.
He told the Commons: “There appears to be no certainty for businesses, for the long-term future of the Good Friday Agreement as clearly any transporting of goods from north to south will need to be checked somewhere and somehow and in echoing the remarks of [May], the continuity of our country as a country that keeps its word and abides by the rule of law and international obligations.
“What certainty can my right honourable friend give me that the government understands the seriousness of these issues?”
Veteran Tory Sir Roger Gale meanwhile warned that the UK’s “honour is not for sale or barter” and that breaking international law would be regarded as “an act of bad faith”.
Responding to May earlier, Northern Ireland secretary Brandon Lewis insisted the provisions were simply a “safety net” to ensure there is no legal confusion if the UK and EU, through the so-called “joint committee” that monitors the implementation of the WA, cannot agree on Northern Ireland before the end of the Brexit transition on December 31.