Theresa May faces a new challenge to her bid to start the process to take Britain out of the European Union after it emerged that opponents plan to launch a fresh legal action on Monday.
Campaigners are planning to write to the government to say that they are going to launch a High Court action to keep Britain in the single market.
The claimants are seeking a judicial review in an attempt to give MPs a new power of veto over the terms on which Britain leaves the EU, following a legal path set by campaigner Gina Miller whose case was heard in the Supreme Court last week.
If they are successful, the legal action could stop the prime minister from making a clean break with Brussels, delay her pledge to kick-start negotiations in March or even derail Brexit completely.
The claimants will argue that the government “has no mandate” to withdraw from the single market because it was not on the referendum ballot paper on June 23and was not part of the Conservative Party manifesto for last year’s general election, it was reported on Saturday.
The legal action bid comes as government sources said they believe there will be a split decision over the Supreme Court’s Article 50 ruling, with judges voting seven to four in favour of giving Parliament a veto on when Britain leaves the European Union, the Sunday Telegraph can disclose.
The news that more judges than expected could back the Government’s argument is a boost for the Brexit side, as some had feared before the case that only one vote would be in the Government’s favour.
A narrow loss for the appeal will make it harder for Remain-supporting MPs and peers to frustrate the progress of a new law to trigger Article 50 of the Lisbon Treaty and start talks to leave the EU by the end of March.
Legal commentators and experts had believed the court comprised overwhelmingly pro-Remain judges, with some expecting the Government would lose by a margin of 10 to one.
However, government lawyers in the courtroom now believe the margin is actually much narrower. One source said: “It is difficult to predict how the case is going to go but the thinking of those in the room is that there might be a sizeable minority who are with the Government.
“The understanding is it is unlikely to be a slam dunk either way; even if a majority agree with Gina Miller [who brought the case] there will be a sizeable minority who don’t. It will potentially be a split decision. The source added: “The feeling among those in the room was the division among the judges was much more pronounced than perhaps it first appeared.”
The Supreme Court is considering the case after losing a High Court hearing last month. The judges are expected to announce their decision before the end of next month.
Miller’s legal team argued that because triggering Article 50 would revoke the legislation which took Britain into Europe, it would require an Act of Parliament to leave the EU.