Boris Johnson has been accused by Tory MPs of “damaging the UK and everything the Conservatives stand for” as he prepares to publish a bill to rip up his 2020 Brexit deal with the EU covering trade with Northern Ireland.

The legislation, to be published on Monday, will bring Johnson into conflict with many of his own Tory MPs, the House of Lords, the EU, lawmakers in Washington and even some business groups in Northern Ireland.

An internal note circulating among Tory MPs opposing the bill and seen by the Financial Times says the measure “breaks international law and no shopping around for rent-a-quote lawyers can hide that”.

The legislation would expunge key elements of the so-called Northern Ireland protocol, part of an international treaty with the EU. It would also give ministers sweeping powers — government officials insist they are just an “insurance policy” — to change almost every aspect of the text.

Brandon Lewis, Northern Ireland secretary, insisted on Sunday the bill was “lawful and correct” and would fix problems in the protocol, part of Johnson’s Brexit deal.

But ministers privately admit the bill could be blocked for months by the House of Lords. Lord Chris Patten, a former Tory chair who led a review of policing in Northern Ireland, said it was “complete madness”.

Johnson argues that the operation of the protocol has created political tensions and business disruption. New checks are needed for goods travelling into the region, which remains part of the EU single market for goods, from the rest of Great Britain.

But the note being shared by Tory MPs, first reported by PoliticsHome, said: “Breaking international law to rip up the prime minister’s own treaty is damaging to everything the UK and Conservatives stand for.”

Government insiders say the bill, drafted in consultation with Tory MPs from the Eurosceptic European Research Group, would fundamentally rewrite the protocol. ERG members warned they would vote against the bill unless it met their demands.

It creates a new regime for border checks: goods from Great Britain destined to stay in Northern Ireland would go through a “green lane” with no checks, while goods heading across the open border into the Republic of Ireland and the EU single market would face “red lane” checks.

The bill would also end the role of the European Court of Justice in policing the protocol, end EU control over state aid and value added tax in Northern Ireland and create a dual regulatory regime, allowing goods originating in Great Britain to circulate in the region provided they meet UK standards, rather than the EU’s.

However the bill also contains a sweeping Clause 15 which would give ministers a reserve power to rip up other aspects of the protocol if it was felt they were causing political or economic disruption in Northern Ireland.

Allies of Liz Truss, the foreign secretary who is sponsoring the bill, insisted this was a technical “insurance” clause to be used as a tidying-up exercise; sceptics at Westminster fear it could be used much more widely.

Whitehall insiders said officials had been left staggered by the scope of the new powers. One former cabinet minister said the proposals showed “utter contempt for the people of Northern Ireland”.

Some Tory MPs fear it could be used to scrap the democratic “consent vote” on the protocol that is set for 2024, where the people of Northern Ireland could decide whether to continue with it.

But government officials insisted that was neither the intention of Clause 15 nor a possible outcome, since the consent vote was enshrined in an international treaty and could not be affected by a domestic law change.

The clause, seen by the FT, would specifically protect only three parts of the protocol, covering rights of individuals, free travel and north-south co-operation in areas like health and agriculture.

Sir Jonathan Jones, the former head of the UK government’s legal department who quit last year over the government’s handling of the protocol issue, said: “They signed a binding international agreement and cannot turn off those obligations just by changing domestic law.”

Meanwhile Johnson’s argument that the protocol is damaging the economy has been contradicted by Northern Ireland’s food, meat and dairy industry in recent days which have asked for it to be retained, arguing it provided them with value access to international markets.