Senior Brexiteers have warned Boris Johnson that key parts of his Withdrawal Agreement with the EU amount to a "poison pill" that should be replaced as part of post-Brexit trade negotiations.
A 120-page report compiled by pro-Leave MPs and lawyers states that exiting the transition period with the current provisions of the agreement in place would have "crippling" consequences for the UK and prevent the country from becoming a "fully sovereign state".
The document, which is published as the UK and EU carry out intensive trade negotiations, has been endorsed by a series of senior backbenchers, suggesting Mr Johnson could face resistance in the Commons if he fails to tackle some of their concerns.
On Saturday, Mark Francois, the chairman of the influential European Research Group (ERG) of Eurosceptic Tory MPs, said: "The report argues that the remaining elements of the Withdrawal Agreement after we leave the transition period cannot be allowed to stand as they are, and particularly that there must be no remaining role for the European Court of Justice over any aspect of our national life. That is something that I and my colleagues in the ERG would very much support."
The report, published by the new Centre for Brexit Policy, includes contributions from Lord Trimble, the former first minister of Northern Ireland, Martin Howe, the Brexiteer QC, and Owen Paterson, the former cabinet minister who chairs the think tank.
The key elements it says make up the "poison pill" include the UK having to remain bound to some state aid laws, the creation of "burdensome EU customs mechanisms" at a border in the Irish Sea, a role for the European Court of Justice (ECJ) for another eight years, and the vast divorce payments, for amounts the report states are "not owing under international law" and are "subject to the determination of the ECJ".
The report states: "Although the Government sees the revised Withdrawal Agreement (WA) as only transitional until the end of the transition period in December, there remain serious threats to UK sovereignty that will have crippling economic and strategic consequences for years to come if they are not dealt with now.
"Exiting the TP with these threats still in place will not return the UK to a fully sovereign state and is unacceptable."
The report urges Mr Johnson to replace the Withdrawal Agreement with a "sovereignty compliant" agreement. A chapter by Lord Trimble states that the current deal "rips the Good Friday Agreement apart” by handing law-making power over Northern Ireland to the EU.
The paper's recommends returning to the Brexiteers' plan of "alternative arrangements" to achieve an "invisible border" on the island of Ireland and using independent adjudication.
A government spokesman said: “The Government’s approach ensures businesses in Northern Ireland will have unfettered access to the rest of the UK market and that there will be no tariffs on goods remaining within the U.K. customs territory and no new customs infrastructure.
“Crucially, at the heart of our proposals is a consensual, pragmatic approach that will protect the Belfast (Good Friday) Agreement and the huge gains from the peace process.”