Arguing the vote for Brexit, which was opposed by world leaders such as Barack Obama and Christine Lagarde of the International Monetary Fund, is far from guaranteed, the group advised the Government to carefully consider whether to follow through with the British public's vote to leave the European Union.
Before Article 50 of the Lisbon Treaty – the route via which members states leave the EU – can be triggered, they say primary legislation will need to be enacted.
The lawyers add that the Government should organise an independent investigation into the costs and benefits of withdrawal before making plans to exit the bloc.
They say their letter will reach Mr Cameron this week and proposes a way forward which "reconciles the legal, constitutional and political issues" triggered by the Brexit vote.
Philip Kolvin QC, who co-ordinated the creation of the letter, said: "Parliament is sovereign and the guardian of our democracy.
"MPs are elected to exercise their best judgment on the basis of objective evidence, to safeguard the interests of the country and their constituents for this and future generations.
"At this time of profound constitutional, political and possibly social and economic crisis, we look to them to fulfil the responsibility placed upon them."
David Lammy, the MP for Tottenham and former Higher Education and Skills Minister, told The Independent: "In our democracy parliament is sovereign - we do not have government by referenda or plebiscite.
“Whoever replaces David Cameron a Prime Minister will have to seek the approval of Parliament before any move is made to invoke Article 50 and trigger Brexit.
“I'm absolutely clear that in the best interests of my constituents and of the nation as a whole I will be voting against the invoking of Article 50 when it comes before Parliament and I know that many of my colleagues share the same view."
Aidan O'Neil, a constitutional and EU law specialist who is one of the signatories, said: "The Brexit referendum has made clear that the UK is not a united nation-state, but a divided state of nations.
"But it has given no mandate or guidance as to what our nations' future relationship might be with Europe, and with each other."
He added: "Precipitate or unilateral action by the UK Government to trigger Article 50 will simply further divide us."
The Leave side won the referendum with a majority of 51.9 per cent, amounting to a lead of more than one million more people.
Read the letter in full:
TO THE PRIME MINISTER AND ALL MEMBERS OF PARLIAMENT
9 July 2016
Dear Prime Minister and Members of Parliament
We are all individual members of the Bars of England and Wales, Scotland and Northern Ireland. We are writing to propose a way forward which reconciles the legal, constitutional and political issues which arise following the Brexit referendum.
The result of the referendum must be acknowledged. Our legal opinion is that the referendum is advisory.
The European Referendum Act does not make it legally binding. We believe that in order to trigger Article 50, there must first be primary legislation. It is of the utmost importance that the legislative process is informed by an objective understanding as to the benefits, costs and risks of triggering Article 50.
The reasons for this include the following: There is evidence that the referendum result was influenced by misrepresentations of fact and promises that could not be delivered.
Since the result was only narrowly in favour of Brexit, it cannot be discounted that the misrepresentations and promises were a decisive or contributory factor in the result.
The parliamentary vote must not be similarly affected. The referendum did not set a threshold necessary to leave the EU, commonly adopted in polls of national importance, e.g. 60% of those voting or 40% of the electorate.
This is presumably because the result was only advisory. The outcome of the exit process will affect a generation of people who were not old enough to vote in the referendum.
The positions of Scotland, Northern Ireland and Gibraltar require special consideration, since their populations did not vote to leave the EU.
The referendum did not concern the negotiating position of the UK following the triggering of Article 50, nor the possibility that no agreement could be reached within the stipulated two year period for negotiation, nor the emerging reality that the Article 50 negotiations will concern only the manner of exit from the EU and not future economic relationships.
All of these matters need to be fully explored and understood prior to the Parliamentary vote. The Parliamentary vote should take place with a greater understanding as to the economic consequences of Brexit, as businesses and investors in the UK start to react to the outcome of the referendum.
For all of these reasons, it is proposed that the Government establishes, as a matter of urgency, a Royal Commission or an equivalent independent body to receive evidence and report, within a short, fixed timescale, on the benefits, costs and risks of triggering Article 50 to the UK as a whole, and to all of its constituent populations.
The Parliamentary vote should not take place until the Commission has reported. In view of the extremely serious constitutional, economic and legal importance of the vote either way, we believe that there should be a free vote in Parliament.
PHILIP KOLVIN QC
And 1053 others
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