Tory rebels are mounting a bid to give Parliament the power to delay Brexit if a suitable deal if not reached by March 2019.
Former minister Anna Soubry led calls for MPs to be allowed to extend negotiations beyond the official exit day if necessary to prevent Britain crashing out of the European Union without a trade deal.
In a joint article with Labour’s Chuka Umunna, Ms Soubry laid out three tests that the Government must reach to secure the support of pro-EU MPs for key Brexit legislation, which is currently undergoing line-by-line scrutiny in the Commons.
Some 24 Tory MPs are understood to support an amendment to the EU (Withdrawal) Bill by former attorney general Dominic Grieve to offer a “meaningful vote” on the Brexit deal, which will come before Parliament next week.
If backed by Labour, the challenge will pose a major threat to a weakened Theresa May, after she lost her parliamentary majority in the general election.
Writing in The Times, Ms Soubry and Mr Umunna, co-chairs of the all-party parliamentary group on EU relations, said: “Through their elected representatives, the British people must now take control and one way to achieve that is to ensure that parliament will not just debate the withdrawal terms but have the power to determine them in statute and consider other options.”
Their demands include a vote on the final deal before Britain leaves the EU, where MPs are given comprehensive detail of the withdrawal agreements, while the terms of our withdrawal should be approved by way of a legally binding act of parliament.
The pair said: “If more time is required before sufficient detail arises from the negotiations, the government can seek an extension of the negotiation timetable from our EU partners to allow for a vote and our departure after March 2019.”
In a dig at Brexit-backing colleagues, they added: “If the government is serious about reasserting parliamentary sovereignty, it should have no objections to these very reasonable demands.”
In a sign of growing disquiet among pro-EU Tories, a group of 19 MPs – including several former cabinet ministers – also wrote to Ms May to condemn their “highly irresponsible” Brexit-backing colleagues who “seek to dictate terms” which could lead to Britain leaving the EU without a deal.
It comes as the Prime Minister faced a race against time to come up with a suitable plan for the Irish border after Brexit, after her allies in the Democratic Unionist Party (DUP) effectively vetoed draft plans to allow similar regulatory arrangements between Northern Ireland and the Republic.
The DUP – who effectively prop up Ms May’s minority government – refuse to accept any deal that would see Northern Ireland offered different arrangements to the rest of the UK.
If Ms May cannot find a deal acceptable to Brussels, Dublin and Belfast then the UK could be prevented from moving onto vital trade talks until next year.