LONDON (AFP) - Leading Brexit campaigners Boris Johnson and Michael Gove insisted on Thursday that Prime Minister David Cameron should stay in his post regardless of the result of Britain's historic EU referendum.

The Conservative lawmakers had clashed with Cameron, their party leader who led the campaign to stay in the European Union, during a bitter and divisive race.

Johnson, who is widely tipped as a potential successor to Cameron and led the campaign to leave the bloc, was alongside Gove among 84 signatories to a letter to the premier made public after polls closed at 2100 GMT.

"We, who are supporters of Vote Leave and members of the Conservative parliamentary party, thank you for giving the British people a choice of their destiny on June 23, 2016," they wrote.

"We believe whatever the British people decide, you have both a mandate and a duty to continue leading the nation implementing our 2015 manifesto."

The letter was published online by lawmaker Robert Syms, who said he had delivered it to Cameron's Downing Street office on Thursday evening.

Syms said the letter represented two thirds of the Conservative MPs who went against the government in backing the "Leave" campaign.

He said he had not had time to reach all the Conservative MPs that backed a "Leave" vote.

There is a widespread belief that Cameron would have no choice but to resign if he loses the referendum, although a final opinion poll on Thursday pointed to a narrow victory for his "Remain" side.

If he stays in post, the Prime Minister will have a difficult task to reunite his centre-right party.

Cameron, who was re-elected with a small parliamentary majority for his Conservative Party last year, said earlier this week that he would want "star players" in his Cabinet after the vote.

"Clearly over time, whatever the outcome, I have to make sure that I have all the star players on the pitch, that we bring together the strongest possible team in the Conservative Party, in parliament, to get on with the job of governing our country," he told ITV television.

Johnson is not currently in the Cabinet, having only stepped down as mayor of London in May. Gove is justice secretary.