European court of justice’s advocate general says article 50 can be revoked without needing permission of all EU members

Remain campaigners have hailed a European legal opinion that the UK can unilaterally abandon Brexit, saying it greatly boosts their efforts to stop the process of exiting the EU.

A senior legal adviser to the European court of justice (ECJ), Campos Sánchez-Bordona, told the court on Tuesday he believed the UK could revoke article 50 independently, without needing the permission of every other EU member state.

Sánchez-Bordona, the court’s advocate general, said it was essential that MPs knew they could stop the Brexit process if they wished, dismissing the British government’s claims that the issue was hypothetical.

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His opinion is widely expected to be upheld by the ECJ in Luxembourg within the next fortnight, following a long legal battle by a Scottish cross-party group of MSPs, MPs and MEPs, which began last December.

Chris Bryant, a Labour MP campaigning for a second EU referendum who became involved in the legal action in Luxembourg, said the advice meant “the terms of the Brexit debate have fundamentally shifted”.

Bryant said it finally killed off claims by Theresa May that the UK could choose only between her widely criticised deal or no deal at all. Backed by the Scottish Greens, Scottish National party and the Lib Dems, he said it strengthened the case for a second referendum.

“The real choice is now clear: a Brexit deal negotiated by the government, or staying in the EU. It is imperative that the final say on this is handed back to the public because only the people of the United Kingdom can sort this out,” he said.

EU law had previously been silent on whether an article 50 process – the provision of the Lisbon treaty invoked by the UK giving two years’ notice that it intended to leave the union – could be unilaterally revoked or whether it needed unanimous approval from all other member states.

But if the European court endorses the advocate general’s views later this month, it will kickstart a fresh legal battle between UK government lawyers and remain campaigners that is expected to end in the supreme court early next year.

The European court’s opinion is expected to be considered in an emergency session by Scottish judges in Edinburgh before Christmas. If they uphold it, the UK government is almost certain to lodge urgent appeals. The government’s lawyers argue that it breaches parliamentary sovereignty for the courts to tell MPs what they can or cannot vote for.

Downing Street officials downplayed the significance of the legal opinion and insisted the question remained hypothetical, since the UK government had no plans to reverse Brexit. “The position of the government has always been that it will not be revoked. That remains the case,” a spokesman said. “You should be careful on what’s happened today. It’s an opinion from the advocate general; it’s not a final verdict of the court.”

UK government sources believe it strengthens May’s hand. One source said that if the European court’s position increased the chances that Brexit could be stopped, Tory hardliners would be under even greater pressure not to sabotage the prime minister’s deal in next week’s crucial Commons vote.

“It’s just a sideshow,” the source said. There were significant obstacles facing remain campaigners: they would need new legislation to reverse the Brexit process and to set up a referendum, and there was no guarantee that remain would win that vote.

Alyn Smith, an SNP MEP who backed the legal action, said the advocate general’s opinion confirmed its long-held view that Westminster could “stop the clock” on Brexit. “We now have a roadmap out of the Brexit shambles. A bright light has switched on above an exit sign,” he said.