Boris Johnson and Michael Gove have attacked David Cameron's failure to reduce migration in a letter published on 29 May. Gove and Johnson's letter says that the government's 2015 manifesto pledge to reduce migration to tens of thousands was "corrosive of public trust" while the UK remained in the European Union.

The letter published in the Sunday Times challenged Cameron to admit that the government had failed in the manifesto pledge. He also said that the pledge was "plainly not achievable" while the UK stayed in the EU.

"Voters were promised repeatedly at elections that 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," said the letter.

A source at Downing Street said the letter was "a transparent attempt to distract". The letter comes as new polling said that more than nine out of 10 economists believe that Brexit would be harmful for the UK economy.

Johnson and Gove are two of the highest profile Conservative figures campaigning for a Leave vote in the referendum being held on 23 June. The attack has stoked talk about widening rifts leading to a possible split in the party.

The letter also challenges the effectiveness of Cameron's attempt to renegotiate the terms of the UK's membership in the EU, saying, "It's government policy that 'EU migrants should have a job offer before they come here.'" it continues to say. "But the EU did not agree to letting the UK implement that policy during the renegotiation of our membership."

The letter also outlined Johnson's and Gove's concerns over "the impact of free movement in the future on public services", saying, "Class sizes will raise and waiting lists will lengthen if we don't tackle free movement."

In response to the letter, a Downing Street source said: "This is a transparent attempt to distract from the fact that the overwhelming majority of economists and businesses believe leaving the single market would be disastrous for jobs, prices and opportunities for people."