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The argument that Parliament can choose how to be involved in the Brexit process at any time in the next few years does not hold water either, Lord Pannick argues, since Article 50 notification will in effect nullify statutory rights and obligations for good and there will be no turning back from that.

"There is no prerogative power to notify and only an act of Parliament can give such authorisation," he says. 

There is quite a raft of case law being brought out by these top lawyers. 

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Lord Pannick says the government's lawyers, including James Eadie, have been overly reliant on pointing to recent pieces of legislation as evidence of Parliament's unwillingness to limit the executive's powers in international affairs.

He says he wants to turn the argument on its head. 

Parliament never intended prerogative powers to be used in the context of EU membership, he says.

And for MPs and peers to change their mind since 1972, they would have had to make the "clearest of statements" to that effect - which he argues have not been proved by his opponents.

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Warming to his theme, Lord Pannick says whatever interpretation one put on the 1972 European Communities Act, Parliament had "not intended" that the rights it created could be "nullifed by a minister" exercising executive powers.

Lord Wilson intervenes to suggest the QC is using his words carefully and was not asserting that Parliament never intended to say the prerogative should not be used, albeit adding the caveat "am I being too pedantic?"

Lord Pannick replies, very tactfully, "your lordships are never pedantic".

He goes on to say that although it is up to the government to prove otherwise, he is happy to "bear the burden" of showing that Parliament never intended the government to use the prerogative power in this way. 

Lord Pannick says the government's case, as elaborated by the attorney general and chief lawyer James Eadie over the past two days, is "wrong for seven reasons" and he intends to develop them one by one.

He says it is up to the other side to prove Parliament has "clearly conferred" a power on the government to nullify statutory rights deriving from EU membership and to prove that a new "constitutional order" has thereby been created.

He disagrees that the 2015 Referendum Act provides any kind of legal underpinning for the use of executive powers to trigger Article 50. 

And he disputes the scope of the prerogative as argued by the government's lawyers, saying "there is no relevant prerogative power in this context".

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John Larkin has concluded his comments and now Lord Pannick, who is leading the legal team challenging the government's Article 50 case, takes the floor.

He is representing investment manager Gina Miller, who is contesting the right of Theresa May to begin Brexit talks without Parliament's approval.

The QC and crossbench peer starts by saying Ms Miller's fundamental case is that "the prerogative power to enter into and terminate treaties does not allow ministers to nullify statutory rights and obligations". 

Sky News

Joshua Rozenberg told Sky News that we must remember the argument has already happened on paper. 

In the other big Brexit story of the day, Boris Johnson has said 18 months is “ample time” for the UK to negotiate with the EU over its deal for exiting.

Speaking as he arrived at a NATO meeting of foreign ministers in Brussels, the Foreign Secretary said he saw no reason why the UK wouldn’t be ready within the timeframe set out by Michel Barnier.

The EU’s chief Brexit negotiator told reporters today that time would be ‘short’, with less than 18 months to negotiate.

“With a fair wind and everybody acting in a positive and compromising mood, as I’m sure they will, we can get a great deal for the UK and for the rest of Europe,” said Mr Johnson. 

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Northern Ireland's attorney general, John Larkin, is continuing to make his case that none of the legislative or constitutional arrangements underpinning devolution should stand in the way of the UK government triggering Article 50.

The 1998 Northern Ireland Act, which set up the NI Assembly and NI executive, made no mention of the UK's EU membership or gave special protection to Northern Ireland in respect of this continuing, Mr Larkin says.

He says the decision to notify the EU of the UK's intention to leave - the so-called Article 50 trigger - will "amend not a comma or a full stop of the 1998 Act".

As he prepares to cite a number of sections of the Act to back up his point, president Lord Neuberger intervenes to ask whether this is strictly necessary, saying he realises the time available to the NI politician is "attenuated" anyway.

Mr Larkin says he agrees and summarises his position: "Can I simply say the claims that these (provisions) expressly or by necessary implication dislodged the prerogative is defeated by a simple reading of those provisions." 

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Somewhat appropriately, Lord Kerr - the Northern Ireland born judge who served as Lord Chief Justice of Northern Ireland from 2004 to 2009 - has taken up the questioning over previous court judgements relating to NI devolution.

John Larkin, Northern Ireland's Attorney General, is referring to the Good Friday Agreement, the 1998 Northern Ireland Act and the British-Irish Agreement of the same year as he talks about the relationship between the UK and the Republic of Ireland and how the UK's exit from the EU will change that.

Despite Brexit, he maintains that there will still be EU-related matters that the UK and the Irish Republic will still have to deal with. 

Robert Craig, expert in public law from the London School of Economics, has been looking further forward this afternoon to Lord Pannick giving presenting his argument on behalf of Gina Miller and her group. 

"People think he is going to be a flamboyant lawyer because he is excellent, but actually what makes him excellent is the opposite," he told BBC News.

"He is so calm, so clear and so effective in what he says because you can follow precisely what he says the whole time. This is not true of all advocates."

The World at One
BBC Radio 4

Sir Keir Stamer, the shadow Secretary for Brexit is calling on the Government to be more open about its plans for how it would proceed in leaving the EU.

He insists his party does not want to stop negotiations from starting, but explains that the Labour Party motion, which will be debated in the House of Commons on Wednesday, asks for more details on " the basic direction of travel" of the government. Mr Starmer says "we need this plan before Article 50 is invoked".

BBC News Channel

Professor Alison Young, from Oxford University, said the main question with the devolved powers is whether the Sewel convention stands in court.

"Conventions are normally political rules and guidelines that are politically enforceable," she told BBC News, "so you could be criticised in parliament for not following them, but the courts will normally say these are not legally enforceable.

"The additional argument though is somehow the Scotland Act will add something into this to make it more legally enforceable." 

It is down to the 11 judges to decide. 

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The justices have returned from lunch and without any preamble John Larkin, the Northern Ireland Attorney General is invited to address the court. 

He starts by saying he realises time is "tight" and he will rely on written submissions to make some of his lesser arguments.

He says he will focus on the status of Northern Ireland within the UK and how that impinges on the UK government's powers vis a vis EU membership. 

Lord Hughes says he is trying to catch up with Northern Ireland documents in the bundle and asks whether they are in the "memory stick". Lady Hale, as she has done before, intervenes to clarify and help out one of her colleagues.

The cameras are rolling again and we expect the judges to come back imminently for the next session to begin.

Dominic Casciani
Home Affairs Correspondent

So what does the government’s case amount to? Quite simply today it came down to a series of bullet points from James Eadie QC 

The prerogative - power of ministers - to make or unmake treaties exists today as a key part of our constitution. Mr Eadie said that Parliament well knew this in 1972 and again in 2015 when it passed the EU Referendum Act. 

But he went further. He argued that the case against the government was a "constitutional trap" that would take the court “over the line” - effectively warning that it would be over-stepping its powers into the realms of politics. Parliament, “does not seem to want the obligation” to trigger Article 50. 

Some key points followed from Lord Keen, the Westminster government’s chief lawyer on Scottish legal issues on whether or not there was some kind of devolution lock.  

Some argue Article 50 can’t be triggered without consent from the devolved bodies. But Lord Keen said today that’s nonsense because those bodies have no responsibility for international relations - including the making and breaking of treaties such as EU membership. 

More after lunch from the Attorney General for Northern Ireland and later this afternoon we finally get to hear a response to the government from Lord Pannick QC, representing Gina Miller, whose challenge is at the heart of this fascinating legal debate.