Chuka Umunna, former Labour shadow business secretary and chairman of Vote Leave Watch, says he's not prepared to give the Conservatives a blank cheque during Brexit negotiations.

He said:

Image source, Reuters

What must the Supreme Court decide?

The government was appealing against a challenge won by two claimants - investment banker and philanthropist Gina Miller and London hairdresser Deir Dos Santos.

In the High Court, they jointly challenged the government's presumption that it had the right to trigger "Article 50", the exit clause in the treaty that makes the UK a member of the European Union.

They argue, instead, that it is for Parliament to say what happens following the Brexit referendum vote.

The Supreme Court's eventual judgement, whatever it may be, will not overturn the referendum result: it will simply determine which course towards exit is lawful.

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Stephen Gethins, SNP spokesperson on Brexit in the House of Commons, says he is hoping the Scottish Parliament, Westminster and other legislatures in the UK will have a say in what happens with Article 50. 

He thinks there should be full scrutiny from legislatures around the UK as would happen with any significant change going through parliament.  

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Iain Duncan Smith, a prominent Brexit campaigner and former Conservative work and pensions secretary, says the Supreme Court risks a "constitutional crisis" if it tries to tell Parliament what to do over Brexit.

"It's not the right of judges to tell Parliament how to go about doing its business," he told the BBC.  

"The question will be: do they just support the High Court or do they go further and say Parliament has to enact legislation?"

Mr Duncan Smith said the issue was already in the territory of a constitutional crisis. 

"This is on the margins of debate about who is supreme," he said.

Even if the Supreme Court judges go against the government, Mr Duncan Smith says he does not think it will have a massive effect in terms of Theresa May's ability to trigger Article 50.

Any bill will only have one or two clauses, he said. "The prime minister will meet her deadline of triggering Article 50 by the end of March."

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The expectation in government circles and among legal analysts is that the Supreme Court will say the prime minister will need the consent of Parliament before formal negotiations on Brexit can begin, BBC political correspondent Carole Walker says.

She says ministers do not think MPs and peers will try to block Brexit, but there will be attempts to amend any legislation on it.

One thing that needs to be decided is how much the government will need to consult the devolved administrations in this process, she says.

Former Lib Dem leader Nick Clegg says while it's reasonable to say the process was kicked off by the British people, he believes it should be signed off by Parliament.

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Ken Clarke MP says lots of Conservative MPs agree parliament should have say on Article 50

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Theresa May's approach to Brexit risks making her the "unwitting tool" of aggressive nationalists seeking to tear the European Union apart, former deputy prime minister Nick Clegg will warn.

Speaking ahead of the prime minister's talks with Donald Trump in Washington on Friday, Mr Clegg will name the new US president as part of an "axis of aggressive nationalism" stretching from the White House to the Kremlin and taking in hardline Brexiteers in the UK and populist parties in countries across Europe.

At a conference in Brussels on Tuesday, the former Liberal Democrat leader will warn Mrs May to put Britain's interests first at the White House on Friday, and not to be drawn into the president's "nationalist tactics to undermine the EU".

He will accuse Mr Trump of "effectively colluding" with Russian leader Vladimir Putin to undermine the security and prosperity of Europe, said party aides.

Mr Clegg is expected to tell the E!Sharp conference: 

Image source, Supreme Court

The Supreme Court is due to rule on whether Parliament or ministers have the power to begin the Brexit process.

The landmark judgement, external, to be announced at 09:30 GMT, will decide how the EU's Article 50 exit clause is triggered.

The government argues that ministers have the power - but opponents say they need Parliament's approval to start the two years of negotiations.

If ministers lose the appeal, they will have to ask Parliament to vote on legislation that triggers Article 50.

The outcome of the Supreme Court case, heard by all 11 justices over four days in December, will not overturn the referendum result but determine which course towards leaving the EU is lawful.

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