Boris Johnson is being warned that he will repeat the mistakes of Partygate by backing “rule-breaking over the rule of law”, when he publishes plans on Monday that are expected to prompt a new Tory rebellion over Brexit.

Frantic legal and political negotiations have been taking place this week among Johnson, his cabinet and MPs in advance of the government’s bill designed to improve trade between Northern Ireland and the rest of the UK. The legislation will be published on Monday.

The government had received legal warnings that any plan that overrides the existing agreement with the EU – the Northern Ireland protocol – could be deemed illegal. However, despite claims that the bill had been watered down, two sources told the Observer that it still contains clauses that effectively sideline parts of the existing agreement.

Tensions are now running high and publication of the bill is likely to reopen the Brexit issue among Conservatives. One Tory source said: “The government is lying to its own MPs and the media about the illegal focus of this bill. The Tory party is sleepwalking into a repeat of the Owen Paterson vote and Partygate – yet again positioning the party full square in support of law-breaking over rule of law.”

It comes after days of claim and counterclaim over the contents of Monday’s bill. Some insiders claimed that chancellor Rishi Sunak and levelling up secretary Michael Gove had successfully prevented the proposals from becoming too hard line and provoking a fierce response from the EU. There have also been claims that Sir James Eadie, the first Treasury counsel, had not been asked for his legal opinion on key elements of the proposals. Whitehall sources insisted he was “happy” and now overseeing all the legal issues relating to the bill.

However, others with knowledge of the discussions said that key clauses were still included that effectively override the existing agreement – a move that will anger Brussels and cause a major rebellion on the Tory benches. Labour has already said it will oppose attempts to override the existing Northern Ireland deal.

There is still a row taking place over what legal advice the government has received over its proposals. Sources told the Observer that key elements of legal advice were not presented to the cabinet committee overseeing the plans. There are also claims that both Sunak and Gove were bypassed and prevented from seeing the final version of the legislation, though this is disputed by government sources.

It has also been established that views on the legality of the proposals were taken from Tom Grant, an academic lawyer who worked in the US state department during the Trump administration. He has been advising on the bill and has previously written legal articles supporting a very hard Brexit. Whitehall sources said while he offered advice, he was not a central part of the legal team.

Alistair Carmichael, the Lib Dem MP who has been asking Commons questions about the legal advice relating to the plans, said: “It’s a sad indictment of this Conservative government’s desperation to justify their reckless plan that they are taking advice from a former aide to Donald Trump. The government must finally come clean on its attempts to shop around for bargain-basement legal advice. We need full transparency over what advice was given and who from.”

Some moderate MPs said they had been assured that Sunak and Gove had made sure the proposals are not too hardline. “What will emerge won’t enrapture [pro-Brexit Tory group] the ERG,” said one source.