LONDON: Two leading members of the ruling Conservatives accused British Prime Minister David Cameron on Sunday of breaking his promise to curb immigration, stepping up hostilities in the party over a battle to win next month's referendum on remaining in the EU.

In an open letter to Cameron, his one-time allies Justice Minister Michael Gove and former London mayor Boris Johnson said "a failure" to curb migration was "corrosive of public trust in politics".

Cameron is leading a campaign to persuade voters to keep Britain in the European Union in the June 23 referendum and the "Remain" side said the attempt to move the debate onto immigration showed that "Leave" campaigners had lost the argument on the economy.

In the letter circulated by the "Vote Leave" campaign, Gove, Johnson and Gisela Stuart, a member of the opposition Labour Party and fellow campaigner, said voters had been promised that annual net immigration could be cut to the tens of thousands.

"This promise is plainly not achievable as long as the UK is a member of the EU and the failure to keep it is corrosive of public trust in politics," they wrote.

They pointed to official statistics issued last week showing net migration to Britain reached 333,000 in 2015, the second-highest level for a year since records began in 1975. Of those, a net 184,000 came from the EU, which upholds the principle of free movement.

Immigration is one of the key battlegrounds in what is becoming an increasingly bitter fight over EU membership, with many voters concerned about the strains a growing number of people put on schools, hospitals and housing.

Those campaigning to stay in the European Union said the "Leave" campaign was struggling after losing the argument on the economy and challenged campaigners to describe what Britain would look like if the country left.

"The reason why the Leave people have now really focused on immigration day after day after day is because they have lost comprehensively the debate on the economy," former Labour prime minister and "Remain" campaigner Tony Blair told the BBC.

"What is now clear ... is that if we did vote to leave the economic aftershock would be severe," he said.

ECONOMICS WON?

Earlier on Sunday, a poll suggested that nine out of 10 of Britain's top economists believed the economy would be harmed if Britain left the EU.

Cameron hailed the results as proof that the overwhelming view was that a British exit would hurt the economy, but the "Leave" campaign, which denies it has lost the argument, cast doubt over whether such a "cosy consensus" should be trusted after many supported scrapping the pound 15 years ago.

With his party split over the EU, Cameron could face a revolt after the referendum.

One Conservative lawmaker, Nadine Dorries, told ITV television her "letter is already in" to call on the prime minister to stand aside after the vote.

Meanwhile, a minister in Cameron's cabinet, Priti Patel, wrote an article accusing the "Remain" campaign of being led by people whose wealth protected them from the impact of immigration.

But former senior minister Iain Duncan Smith, who is campaigning for an exit, said "Leave" should focus on the debate.

"I am not going to be in favour of changing the prime minister at this particular point or at any stage," he told ITV. "I have always said if he was to stand again I would support him."