Parliament must vote before the Government formally triggers the exit procedure from the EU, a top law firm has insisted.
Mishcon de Reya, acting on behalf of an anonymous group of clients, has taken legal action to ensure MPs have their say before Downing Street invokes Article 50 of the Lisbon Treaty which formally sets withdrawal in train.
Outgoing Prime Minister David Cameron has insisted it will be the duty of his successor to trigger the mechanism after he declined to do so in the wake of the 52% to 48% vote to leave the EU.
Front-runner to take over from Mr Cameron, Home Secretary Theresa May, has suggested it will be the end of the year before she pushes ahead with Article 50 if she takes the Tory crown.
The legal firm has insisted that the decision to proceed with moving Article 50 rests with Parliament following the referendum, which some Remain MPs have stressed was advisory and not binding on the legislature.
With a large majority of MPs supporting the Remain cause in the referendum campaign, some Brexit backers have issued warnings that Parliament must accept the outcome of the national vote.
The legal firm is adamant that Parliament must have its say, and has been in correspondence with government counterparts to seek assurance over the process and plan to pursue it through the courts if they are not satisfied.
Kasra Nouroozi, a Mishcon de Reya partner, said: "We must ensure that the Government follows the correct process to have legal certainty and protect the UK Constitution and the sovereignty of Parliament in these unprecedented circumstances.
"The result of the Referendum is not in doubt, but we need a process that follows UK law to enact it.
"The outcome of the referendum itself is not legally binding and for the current or future prime minister to invoke Article 50 without the approval of Parliament is unlawful.
"We must make sure this is done properly for the benefit of all UK citizens. Article 50 simply cannot be invoked without a full debate and vote in Parliament.
"Everyone in Britain needs the Government to apply the correct constitutional process and allow Parliament to fulfil its democratic duty which is to take into account the results of the referendum along with other factors and make the ultimate decision."
EU governments have put pressure on London to invoke Article 50 as soon as possible to ensure a speedy withdrawal as the negotiating process takes up to two years.