The court of session in Edinburgh has said a legal action can go ahead to establish whether the UK can unilaterally stop the Brexit process if British voters decide the final deal is unacceptable.

The petition, brought by a group of MPs, MEPs and MSPs from four parties in Scotland, excluding the Conservatives, will now be served on the UK government, which has 21 days to respond.

The seven 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 first four signatories were Alyn Smith, an SNP MEP; Ross Greer MSP and Andy Wightman, two MSPs from the Scottish Greens; and David Martin, a Scottish Labour MEP.

Martin has now been joined by the second of Labour’s two Scottish MEPs, Catherine Stihler, adding to the divisions within Labour over Jeremy Corbyn’s policy that the leave vote should be honoured. Corbyn’s stance has been endorsed by Richard Leonard, the recently elected Scottish Labour leader.

Christine Jardine, a Liberal Democrat MP for Edinburgh West, and Joanna Cherry, an SNP MP, QC and former prosecutor, are also signatories.

Quick Guide What are Brexit options now? Four scenarios Show Staying in the single market and customs union The UK could sign up to all the EU’s rules and regulations, staying in the single market – which provides free movement of goods, services and people – and the customs union, in which EU members agree tariffs on external states. Freedom of movement would continue and the UK would keep paying into the Brussels pot. We would continue to have unfettered access to EU trade, but the pledge to “take back control” of laws, borders and money would not have been fulfilled. This is an unlikely outcome and one that may be possible only by reversing the Brexit decision, after a second referendum or election. The Norway model Britain could follow Norway, which is in the single market, is subject to freedom of movement rules and pays a fee to Brussels – but is outside the customs union. That combination would tie Britain to EU regulations but allow it to sign trade deals of its own. A “Norway-minus” deal is more likely. That would see the UK leave the single market and customs union and end free movement of people. But Britain would align its rules and regulations with Brussels, hoping this would allow a greater degree of market access. The UK would still be subject to EU rules. The Canada deal A comprehensive trade deal like the one handed to Canada would help British traders, as it would lower or eliminate tariffs. But there would be little on offer for the UK services industry. It is a bad outcome for financial services. Such a deal would leave Britain free to diverge from EU rules and regulations but that in turn would lead to border checks and the rise of other “non-tariff barriers” to trade. It would leave Britain free to forge new trade deals with other nations. Many in Brussels see this as a likely outcome, based on Theresa May’s direction so far. No deal Britain leaves with no trade deal, meaning that all trade is governed by World Trade Organization rules. Tariffs would be high, queues at the border long and the Irish border issue severe. In the short term, British aircraft might be unable to fly to some European destinations. The UK would quickly need to establish bilateral agreements to deal with the consequences, but the country would be free to take whatever future direction it wishes. It may need to deregulate to attract international business – a very different future and a lot of disruption.

A crowdfunding campaign to meet the legal costs has passed its £50,000 target, and the group has now filed its petition to the court of session asking judges to refer the issue to the ECJ in Luxembourg.

Michel Barnier, the EU’s chief negotiator, told the Guardian this week that any decision to abandon Brexit needed an agreement from all 27 but the petitioners say that position has never been tested by the courts.

Jolyon Maugham QC, who is coordinating the legal action through his campaign group the Good Law Project, said he believed there was a clear case for giving the UK parliament the right to unilaterally cancel the Brexit process.

It would strengthen Westminster’s ability to abandon Brexit if the deal was so poor it was against the national interest; it would also strengthen the UK government’s hands in the negotiations to know it could withdraw its article 50 letter without needing agreement from all 27 members.

In a significant boost to their case, one of the EU’s most senior legal experts argued the UK could withdraw article 50 without the consent of other member states.

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

art 50 is based on the principle that withdrawing from the EU is a unilateral decision. Nobody can force a state to leave.nobody can prevent it to leave. The only condition is that it’s decision is taken in conformity with its constitutional requirements. At any moment may change — JC Piris (@piris_jc) December 20, 2017

Some EU diplomats agree with Maugham and the petitioners that the question can only be resolved by the ECJ, amid uncertainty about the law. Article 50 is silent about whether it can be revoked before the withdrawal day.

“I know a lot of colleagues who say: ‘[Brexit] is so bad and I wish they hadn’t done it,’ so my working assumption is that there would be unanimity [for the UK to remain],” said one senior diplomat. “But whether the UK can backtrack unilaterally, that I don’t know. That would deprive the 27 of the possibility to extract some concessions.”

In its response to the initial letter advising it of the legal action, the UK government has denied it has formally supported Barnier’s stance that the article 50 process cannot be stopped unilaterally.

The office of the advocate general, the UK government’s Scottish law officer, said: “For the avoidance of doubt, the public position we do recognise having taken is that the stated and consistent position of the government has been and is that the UK’s notification under article 50 (2) will not be withdrawn. This is clearly long established and consistent and discloses no basis for legal challenge.”

The seven politicians dispute that, citing statements by David Davis, the Brexit secretary, and Martin Callanan, the Brexit minister in the Lords, in parliament that article 50 cannot be revoked.