Theresa May will set a "disturbing precedent" if she triggers the mechanism to officially split from the EU without seeking parliamentary approval, an influential group of peers have warned.

The House of Lords Constitution Committee also argued that it would be "constitutionally inappropriate" if the prime minister fails to give MPs a say over Article 50.

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"The referendum result was clear and it is right that the Government are preparing to take Britain out of the EU," said Lord Lang of Monkton, chairman of the committee.

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

"Parliament should be asked to approve the decision to trigger Article 50 — a decision that will start the formal process of the UK leaving the EU and set a deadline for the UK's exit.

"Parliament's assent could be sought by means of legislation or through resolutions tabled in both Houses of Parliament."

But the Conservative premier has already ruled out giving parliament a say on the mechanism, which is expected to start two years of Brexit negotiations with the EU.

"Both Houses of Parliament decided to put the decision about whether or not we remain a member of the EU in the hands of the British people and we now need to get on with delivering that decision," a spokeswoman for May said this afternoon (13 September).

Cambridge University's Professor Kenneth Armstrong, an expert in European law, has backed May's decision. The academic argued that the EU referendum provided a non-binding vote to advise the government, rather than parliament, and the executive should therefore take the lead.

"Who exactly is the electorate advising? It cannot really be parliament because the electorate gives its input to parliament through the medium of the election of MPs and their representative role within parliament," he said.

"This must, instead, be more in the nature of advice to government. And the decision of the electorate that the UK should leave the European Union is its advice to government which it should act upon. For parliament to frustrate that would be to deny the whole point of the exercise."

The row comes as senior MEP Guy Verhofstadt, who is leading Brexit negotiations for the European Parliament, called for the UK to break away from the EU before 2019.

The former prime minister of Belgium also warned that the UK cannot have what EU chiefs have dubbed as "single market a la carte".

"If [the] UK wants access to the single market, it must also accept the free movement of citizens. Our four freedoms are inseparable," Verhofstadt said.

But May has ruled out triggering Article 50 this year, instead the government is expected to make the move next year.

Brexit Theresa May EU Referendum