A legal action to decide if Britain can unilaterally halt Brexit if MPs vote down Theresa May’s withdrawal deal should go ahead, a court has ruled.

The Government will now be required to respond to a petition, brought by a cross-party group of Scottish politicians, after the decision in Edinburgh.

The politicians want the European Court of Justice (ECJ) to rule on whether the UK can halt the Article 50 process without needing the approval of all other 27 EU member states.

The move comes as one of the EU’s most senior legal experts boosted the case by arguing the UK could withdraw the exit notification without that consent.

“Article 50 is based on the principle that withdrawing from the EU is a unilateral decision. Nobody can force a state to leave,” Jean-Claude Piris, former head of the EU council’s legal service, wrote on Twitter.

“The only condition is that [the] decision is taken in conformity with its constitutional requirements.”

Meanwhile, as The Independent reported, MPs will vote next month in an attempt to lift the lid on secret legal advice which is believed to say that Britain could act alone to halt Brexit.

The Prime Minister has been accused of suppressing a formal opinion that Parliament has the power to reverse the Article 50 notice.

Brexit: the deciders Show all 8 1 /8 Brexit: the deciders Brexit: the deciders European Union's chief Brexit negotiator, Michel Barnier Getty Brexit: the deciders French President Emmanuel Macron Getty Brexit: the deciders German Chancellor Angela Merkel Reuters Brexit: the deciders Commission President Jean-Claude Juncker EPA Brexit: the deciders The European Parliament's chief Brexit negotiator Guy Verhofstadt Getty Brexit: the deciders Britain's Prime Minister Theresa May Getty Images Brexit: the deciders Britain's Chancellor of the Exchequer, Philip Hammond PA Brexit: the deciders After the first and second appointed Brexit secretaries resigned (David Davis and Dominic Raab respectively), Stephen Barclay is currently heading up the position PA

The controversy could be crucial if MPs reject any Brexit agreement she secures in a “meaningful vote” to take place next year, before the withdrawal process can start.

The signatories to the action, at the Court of Session in the Scottish capital, SNP and Liberal Democrat MPs, MEPs representing the SNP and Labour, and SNP and Green members of the Holyrood Parliament.

Brexit: Article 50 author says Theresa May is misleading the public on reversing result

The inclusion of Labour’s two Scottish MEPs, David Martin and Catherine Stihler, adds to party divisions, given Jeremy Corbyn’s stance that the referendum decision should be respected.

A crowdfunding campaign to meet the legal costs has passed its £50,000 target, with a petition asking the Scottish judges to refer the issue to the ECJ in Luxembourg.

Jolyon Maugham, a QC coordinating the action through his Good Law Project campaign group, said it was right for the UK parliament to have the power to cancel the Brexit process.

It would allow Brexit to be abandoned if the Prime Minister’s deal was poor – as well as strengthening her hand in the negotiations in Brussels, he argued.

Mrs May has insisted a rejection of her deal would simply mean Britain crashing out with no deal – insisting the Article 50 process is unstoppable.

But Lord Kerr, a former diplomat and the Article 50 author, argued the British Government could withdraw its letter and stop the process if it wished to.

Separately, the Government is being taken to court over a claim the UK never made the constitutional decision to leave the EU.