Lawmakers in parliament should decide whether Britain leaves the European Union because the Brexit vote was not binding, more than 1,000 prominent British lawyers said in a letter to Prime Minister David Cameron.
The signatories, which include senior lawyers, said that lawmakers should have a free vote in parliament before any British leader takes the decision to trigger the formal EU divorce procedure by invoking Article 50 of the Lisbon treaty.
A separate group of lawyers advising the British government has said the prime minister does not need parliamentary approval to start the process. The government has also rejected a petition for a second referendum.
The letter is the latest attempt by opponents of Brexit to slow the divorce process. Some "Leave" campaigners say there is a concerted attempt by the British elite to prevent an EU departure by entangling any process in political and legal challenges.
"Our legal opinion is that the referendum is advisory," the lawyers said in a letter dated July 9 that was signed by 1054 lawyers. Reuters has a copy of the letter.
"We believe that in order to trigger Article 50, there must first be primary legislation," said the letter which was signed by 118 eminent lawyers known as Queen's Counsel.
Turnout in the June 23 referendum was 72.2 percent. A total of 17.41 million people, or 51.9 percent, voted to leave the EU while 16.14 million, or 48.1 percent, voted to remain in the EU.
While Theresa May, the only Conservative Party candidate remaining to succeed Cameron, says the vote will be implemented, the government is facing several legal challenges over whether it can begin divorce proceedings without approval from parliament.
May has said Article 50 should not be invoked before the end of this year.
The British government rejected an online petition signed by 4.1 million people calling for a new referendum on whether to leave the EU.
"The Prime Minister and Government have been clear that this was a once in a generation vote and, as the Prime Minister has said, the decision must be respected," the Foreign Office said.
"We must now prepare for the process to exit the EU."
A separate set of lawyers, acting for hairdresser Deir Dos Santos, has started proceedings aimed at forcing the government to allow Parliament to decide whether Brexit goes ahead.
The argument for Parliamentary approval before invoking Article 50 is based on a reasoning of the authority for EU law in the United Kingdom. Under this reasoning, EU law applies in the UK because of the European Communities Act of 1972 so the executive cannot undermine rights given by Parliament by triggering Article 50.
"Articles 1 and 2 of the Bill of Rights of 1688, which is still in force today, expressly provides that no personal body apart from Parliament itself can override an act of Parliament and therefore only Parliament can take the decision to withdraw," said Dominic Chambers, a senior lawyer working on the Dos Santos case.
"If the legal challenge is successful, the result will be that there will have to be a debate, a vote in Parliament and then it is up to Parliament to decide," Chambers said by telephone.
Chambers said he was not charging his client.
When asked whether it was an attempt to derail Brexit, Chambers said: "My client respects the results of the referendum and he is not in any way doing this to try to prevent Brexit: His sole concern is upholding the rule of law."
Proceedings were issued on June 28 and a preliminary hearing in the High Court is due on July 19. Santos could not be reached for comment.
The Brexit vote unleashed turmoil in financial markets and has raised the prospect of the division of the United Kingdom as England and Wales voted to leave, while Scotland, Northern Ireland and Gibraltar voted to remain.
The day after the vote, Scotland's first minister, Nicola Sturgeon, said a second independence referendum for Scotland was highly likely.
"The Brexit referendum has made clear that the UK is not a united nation-state, but a divided state of nations," said Aidan O’Neill, who specialises in constitutional law and EU law. "If the UK is to survive the result of this vote, a consensus needs to be built up about the way forward."
"Fully informed discussions and deliberations within and between our parliaments is the only proper constitutional way to achieve this," O’Neill added.
The lawyers who wrote to Cameron, including Philip Kolvin, a senior advocate at Cornerstone Barristers, said the British government should establish a royal commission to review the evidence and to report on the benefits and risks of triggering Article 50.
"The parliamentary vote should not take place until the commission has reported," the letter said.