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

Justice Minister Michael Gove and former London mayor Boris Johnson, in an open letter to the prime minister, who is leading a campaign to persuade voters to keep Britain in the EU, said "a failure" to curb migration was "corrosive of public trust in politics".

In the letter circulated by the "Vote Leave" campaign yesterday, Mr Gove, Mr Johnson and Ms 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 last year, 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.

The "Remain" side, however, said the attempt to move the debate to the immigration issue showed that "Leave" campaigners were struggling after losing the argument on the economy. They also challenged campaigners to describe what Britain would look like if it left the EU.

"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."

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

The poll, which the Observer newspaper said was the biggest of its kind - drawing responses from more than 600 economists - found that 88 per cent of those asked said an exit from the EU and the single market would most likely damage Britain's growth prospects over the next five years.

Mr Cameron hailed the results as proof that the overwhelming view was that a so-called Brexit would hurt the economy, but Mr Matthew Elliott, chief executive of Vote Leave, accused economists of making "the same hollow arguments" they made to support switching to the euro.

"They were wrong then and they are wrong now," he said in a statement.

REUTERS