If Boris Johnson’s government pursues its stated intent to amend part of the Brexit withdrawal agreement regarding arrangements in Northern Ireland, ministers will not only have to persuade a hugely sceptical EU, they must also get a bill through the Commons and the Lords. The UK internal market bill has its second reading in the Commons on Monday. Johnson’s 80-seat majority should ensure it passes, although some Tory backbenchers have spoken out against the breach of international law that it represents. There are many more Conservative MPs who do not like the idea but are staying quiet, in part, they say, in the hope it is all a negotiating bluff by No 10, but also because they fear losing the party whip if they vote against the government, which is the Johnson way. Things are different in the Lords. In a brief debate on the issue on Thursday, Michael Howard became the third former Conservative leader in two days to condemn the promised breach and to warn of the consequences. Lord Howard was more brutal even than Theresa May and John Major as he asked Richard Keen, the Scottish advocate general, who was speaking for the government, about comments in the Commons by the Northern Ireland secretary, Brandon Lewis, who conceded on Tuesday that the new Brexit plans would break international law in a “very specific and limited way”. “Does my noble and learned friend simply not understand the damage done to our reputation for probity and respect for the rule of law by those five words uttered by his ministerial colleague, in another place, on Tuesday – words that I never thought I would hear from a British minister, far less a Conservative minister?” Howard said. “How can we reproach Russia, China or Iran when their conduct falls below internationally accepted standards, when we are showing such scant regard for our treaty obligations?” The Lords debate was brief, but no peers spoke in favour of the government’s plans. It was sought by Charles Falconer, the shadow attorney general, who said the “Lewis mantra” would be used as an excuse by dictators worldwide. Later another senior Tory peer, the former chancellor Norman Lamont, predicted that the bill would not pass through the Lords in its current form. “I think the government had an arguable case, but that case was destroyed the moment Brandon Lewis said the means the government were going to use to change the Northern Ireland protocol was against international law,” he told the BBC. “That is impossible to defend. I think the government are in a terrible mess and in a hole and I don’t think it is easy to justify.” Under the long-established constitutional principle known as the Salisbury convention, peers do not block or severely amend measures that were in the election manifesto of a governing party. Johnson and his ministers could certainly claim that implementing Brexit was not only a manifesto commitment but a central one. However, dissenting peers would stress that breaching international law did not form part of this, and they could return the bill to the Commons without several key clauses. For all the sway that Johnson and his lieutenants have over Tories in the Commons, especially the very Brexit-minded 2019 intake, some more seasoned MPs and peers will listen carefully to the views of Major, who said flouting international law could leave the UK having “lost something beyond price that may never be regained”. Responding to the EU’s pledge to break off talks if the UK proposals were not dropped, Michael Gove, the minister who leads on Brexit issues, said the bill’s introduction would be “an opportunity for the government to set out in detail why it’s important that we have this legislation”. It would need to be an impressive argument to change many minds.