A legal challenge to the government’s right to begin the official process of Brexit without parliamentary approval is due to get under way.
The High Court will consider whether ministers can invoke Article 50 of the Lisbon Treaty, the trigger for formal talks on separation, without MPs passing an act of Parliament.
The case is being brought by investment manager Gina Miller among others.
Ministers argue they can act under ancient powers of royal prerogative.
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The two-day judicial review, due to conclude on Monday, will be heard by the Lord Justice of England and Wales, Lord Thomas.
Prime Minister Theresa May has said she will activate Article 50, formally notifying the EU of the UK’s intention to leave, by the end of next March. This follows the UK’s decision to back Brexit in June’s referendum by a margin of 51.9% to 48.1%.
The EU’s other 27 members have said discussions about the terms of the UK’s exit – due to last two years – cannot begin under Article 50 has been invoked.
Ms Miller is contesting the government’s authority to proceed without recourse to Parliament – arguing the principle of parliamentary sovereignty underpins the constitution and the rule of law in the country.
Her legal team, headed by constitutional lawyer and cross-bench peer Lord Pannick, is expected to argue that invoking Article 50 will threaten the rights of individuals enshrined in the 1972 European Communities Act – the piece of legislation which paved the way for the UK to join the European Economic Community.
Only Parliament, they will argue, can remove or reduce rights granted under law and Article 50 must have the consent of the House of Commons and the House of Lords.
by Clive Coleman, BBC legal correspondent
In today’s constitution the Royal Prerogative is basically a collection of executive powers held by the Crown.
They go back to medieval times but are now placed in the hands of ministers. They’re used, for instance, in some areas of foreign affairs which Parliament has left to the government.
But prerogative powers remain controversial because they’re exercised without any parliamentary authority.
The case has huge constitutional importance and should provide clarity on whether executive powers can, in effect, trump an act of Parliament.
Those bringing the case argue that legislation can only be altered by legislation. The government says it intends to give effect to the outcome of the referendum by bringing about the exit of the UK from the EU.
And that that is a proper constitutional and lawful step to take, using prerogative powers, in light of the referendum result and the democratic mandate it has provided.
Ms Miller, who runs her own investment firm SCM Private, has said the case is in no way an attempt to delay the process of leaving the EU or to overturn the referendum result.
“It is about ensuring for the future of this country that the legally correct process for leaving under the UK constitution is followed,” she said.
“It should be undertaken in accordance with our laws. If we do not have clarity over the correct legal way to trigger Article 50, it could result in significant legal disputes and uncertainty over the validity of the notification”.
Ms Miller has been joined in her legal action by London-based Spanish hairdresser Deir Dos Santos, the People’s Challenge group set up by Grahame Pigney and a campaign group called Fair Deal for Expats.
The government, to be represented by Attorney General Jeremy Wright, is expected to argue it is giving effect to the will of the people provided for in the 2015 EU Referendum Act authorising the poll and that was “clearly understood” before June’s vote.
According to documents published this summer, ministers believe the use of prerogative powers once held by the Sovereign but now residing in the executive to enact the referendum result is “constitutionally proper and consistent with domestic law”.
For the courts to require Parliament to pass legislation to implement the outcome of the referendum would be an “impermissible” intrusion on its proceedings.
“The decision to withdraw from the EU is not justiciable,” they stated. “It is a matter of the highest policy reserved to the Crown.”
The hearing comes amid growing calls from MPs on all sides of the House for the UK’s blueprint for its Brexit negotiating to be subject to far greater parliamentary scrutiny.
While the government has not ruled out giving MPs a vote on the final settlement reached with the EU, it has said on several occasions that it will not countenance a vote on the timing of Article 50 or its negotiating strategy.
The losing side in the case is likely to launch an appeal. It has already been announced that any appeal will be fast-tracked to the Supreme Court to ensure a final judgement before the end of the year.
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