THERESA May should not invoke Article 50 – triggering the UK’s exit process from the European Union – without “explicit parliamentary approval”, an influential House of Lords committee has warned.

In a direct challenge to the Prime Minister, the Lords Constitution Committee declared it would be beyond the scope of acceptable autonomy for the Government to act on “an advisory referendum” without involving Parliament.

Its report, published yesterday, said: “It would be constitutionally inappropriate, not to mention setting a disturbing precedent, for the executive to act on an advisory referendum without explicit parliamentary approval – particularly one with such significant long-term consequences. The government should not trigger Article 50 without consulting parliament.”

Ian Lang, the Conservative peer and chairman of the committee, added: “The referendum result was clear and it is right that the Government are preparing to take Britain out of the EU.

“However, our constitution is built on the principle of parliamentary sovereignty and the decision to act following the referendum should be taken by parliament.”

May has indicated she does not intend to bring the matter before parliament before invoking Article 50 of the Lisbon Treaty, and it is yet unclear when it will be triggered.

Giving evidence to MPs on the Foreign Affairs Committee yesterday, Brexit Secretary David Davis dismissed the suggestions from the Lords committee, insisting there would be no need for parliament to be given a say. He said when ministers initiate the process they will be acting with a mandate from the referendum.

Article 50 itself states: “Any member state may decide to withdraw from the union in accordance with its own constitutional requirements.”

It does not specify what the constitutional requirements are, allowing rival claims of authority to be advanced by parliament and the executive.

In Holyrood today, the First Minister is expected to urge opposition politicians to unite behind her Government’s efforts to keep Scotland in the EU and ensure it remains in the single market.

On a day when the EU focus will dominate activities in the Scottish Parliament, Nicola Sturgeon will appeal to MSPs examining the impact of Brexit on the nation.

Later in the day, Mike Russell, Minister for UK negotiations on Scotland’s place in Europe, will open the first in a series of debates in the chamber establishing key principles for Scotland’s involvement in negotiations.

He is expected to call on the UK Government to end its “confused and damaging” messages, and provide clarity and direction on its plans for leaving the EU, or risk damaging confidence in the UK.

The activity comes on the same day that President of the European Commission Jean-Claude Juncker will make his State of the Union address in Strasbourg outlining his priorities for the year ahead, including how the commission will address the issue of the UK leaving the EU.

During his address, Russell is expected to say: “Scotland’s voice is being heard across the European Union. We want Europe and the world to know that Scotland voted wholeheartedly to remain in the EU as befits a nation that is outward-looking and open.

“The message from the United Kingdom Government has been both confused and damaging.

“At times it seems as if there is no direction.

“At others it seems as if a hard Brexit, with all the damage to jobs that would cause, is the desired destination.

“We have no idea whether or not the UK wishes to remain within the single market, almost three months on from the referendum. That is not good enough.

“I am today inviting this Parliament to add their voice to ours in holding the UK Government to the Prime Minister’s commitment and reiterating their support for our principled and values-driven approach to protecting Scotland’s interests, fundamental to which is to remain inside the single market.”

People in Scotland voted by 62 per cent to 38 per cent to remain in the EU. However, the country risks being pulled out of the bloc as the UK overall voted by 52 per cent to leave.

The divergent result has been picked up on by European politicians – perhaps most significantly by Guy Verhofstadt, who was appointed lead Brexit negotiator for the European Parliament last week, and has said it would be wrong for Scotland to have to leave the EU after it voted to remain.

He has also said he believes there would be no obstacles to an independent Scotland joining the bloc.

Earlier this month, a report by the Scottish Government said the projected cost to the Scottish economy of leaving the EU would amount to up to £11.2 billion per year by 2030.

Meanwhile, yesterday Nick Clegg, the former Lib Dem leader and former deputy prime minister, said he expects Liam Fox, the international trade secretary, to resign within the next 18 months.

He said: “I do feel sorry for Liam in particular. I’m not a betting man, but if I was I would put a fair amount of money that Liam Fox will resign in a huff within the next 18 months. He doesn’t have a job and he doesn’t appear to have realised that yet.”

Clegg explained: “If the United Kingdom doesn’t leave the customs union, which apparently is still an open question in Whitehall, then he is heading a department without purpose, because he cannot negotiate all these fantastic trade deals with Papua New Guinea and Tanzania and China and India and Australia.”