The government has warned a group of MPs they are breaching parliamentary privilege by asking a court to rule on whether Brexit can be halted unilaterally by the UK.

Three MPs are involved in a legal case brought by a group of eight pro-remain MPs, MSPs and MEPs, who want the European court of justice to rule on whether the UK can abandon moves to leave the EU without the consent of other member states.



Their lawyer, Aidan O’Neill QC, told a civil court in Edinburgh on Tuesday there was a clear and urgent need for the court to refer the case to the European court, because MPs were soon to start voting on the EU withdrawal bill.



His application was challenged by David Johnston QC, for the UK government. Johnston told Lord Boyd that MPs had no constitutional right to ask a court to interfere in a political decision at Westminster.



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Implying that Lord Boyd would be acting unlawfully if he supported the application, Johnston said the UK constitution laid down very clear rules on the separation of powers between parliament, the government and the judiciary.

He said the MPs were effectively asking the courts for legal advice, which could only be given in exceptional circumstances.

“Constitutionally [it] is not a matter for individual members [of parliament] to seek to enlist the assistance of the courts for the purposes of conducting parliamentary business,” Johnston told the court of session hearing.

“When the petitioners seek the assistance of this court, in order to assist them in deciding how to cast their votes, their request involves a violation of parliamentary privilege.”

The UK government had repeatedly stated it would not halt or reverse Brexit and there was no evidence that other parties at Westminster would attempt to do so either, he added.

Ministers only wanted to give MPs two options: to accept the deal or vote for Brexit without one. Therefore the question of whether article 50 could be unilaterally revoked was hypothetical and academic, he said.

Quick guide What is the EU withdrawal bill? Show Hide What is the EU withdrawal bill? The EU Withdrawal Bill – once known as the Great Repeal Bill – is going through the House of Commons to repeal the 1972 European Communities Act and transpose all existing EU legislation into domestic UK law, which will avoid a 'cliff-edge' change on the day after we leave the EU. Parts of the bill have been highly controversial, and MPs have tabled hundreds of amendments to try and change its wording, including a significant number of Conservative rebels. Some of the key controversies include its use of so-called Henry VIII powers, which will give government ministers the power to tweak the wording of laws to make sure they make sense in UK legislation - but those changes could take place without having to go through parliament. MPs have called this a "power grab" by the government. The government estimates around 800 to 1,000 measures called statutory instruments will be required to make sure the bill is applied correctly. Other concerns include the government's decision not to include the EU charter of fundamental rights in the law being transposed. Other amendments are attempts to affect the Brexit process, including legislating for a transitional period and giving MPs a binding meaningful vote on the deal secured by Theresa May, before the deal is finalised.

The petition was lodged last year by two Scottish Green party MSPs, Andy Wightman and Ross Greer; the Scottish National party MP Joanna Cherry QC; the SNP MEP Alyn Smith; the Scottish Labour MEP David Martin; and the Scottish Liberal Democrat MP Christine Jardine.

Jardine has withdrawn but two English MPs have since joined as interested parties: Tom Brake for the Lib Dems and the pro-remain Labour MP Chris Leslie. This is the third hearing on the case, after an earlier attempt was rejected by another judge, and then appealed against by the cross-party group.

Article 50 of the European treaty has been invoked by the UK government to trigger withdrawal from the EU but that article is silent on whether the member state that has triggered it unilaterally can also cancel it unilaterally.

UK ministers and the European commission have both indicated they believe that withdrawing an article 50 application requires the consent of all 27 other EU member states. Other EU legal experts say that is wrong, but only the European court of justice can offer a definitive and binding decision.

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O’Neill told the court the EU withdrawal bill clearly mandated Westminster to vote on both the Brexit deal and on enacting Brexit, but also to vote for a third option that contradicted government policy.

If no deal was reached by 28 February 2019, MPs were empowered by the bill to instruct the government to follow its votes.

O’Neill said MPs needed clarity on this EU law so they had all the information they needed when they voted. “They need to know so they can exercise their responsibilities as legislators,” he told Lord Boyd.

The hearing is expected to last one day, with Lord Boyd due to give his ruling later. Jolyon Maugham QC, the lawyer who helped arrange the case after a crowd-funding appeal, has said the cross-party group is likely to appeal against any ruling against them.