article: Michael Gove and Boris Johnson said the pledge was "corrosive of public trust" while Britain remained in the EU. But Number 10 said their claim was an "attempt to distract" from the fact that an EU exit would be "disastrous". It comes amid growing Tory turmoil over the EU, with some MPs threatening a post-referendum leadership challenge. Meanwhile, a survey of economists has suggested that nine in 10 of those who responded believe leaving the EU would be damaging. There are less than four weeks to go until the UK decides whether to stay in or leave the European Union on 23 June. John Pienaar: Tory division escalates EU referendum: All you need to know Issues guide: Explore the arguments Immigration is one of the key battlegrounds in the referendum debate - and it is the focus of an open letter to Mr Cameron, published in the Sunday Times, by Mr Gove and Mr Johnson, who say the government's pledge to get migration below 100,000 was unachievable. Net migration - the difference between the number of people coming to the UK for at least a year and those leaving - rose to 333,000 in 2015, according to Office for National Statistics estimates. The figure for EU-only net migration was 184,000, equalling its record high, and 188,000 for non-EU. This issue covers immigration and free movement within Europe. Mr Johnson and Mr Gove said: "Voters were promised repeatedly at elections that net migration could be cut to 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 also said they were "particularly concerned about the impact of free movement in the future on public services". "Class sizes will raise and waiting lists will lengthen if we don't tackle free movement," they wrote. In response to the letter, Number 10 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." By John Pienaar, deputy political editor The Brexiteers point is that EU open borders make immigration control impossible. Its effect, though, is to accelerate a descent into internecine warfare which now threatens to make the Conservatives ungovernable if the referendum ends in anything but a decisive victory for the Remain campaign. So bitter has the conflict become, so taut the tension between the rival factions, that angry Eurosceptic Tories talk privately of challenging the prime minister's position even if Britain votes to stay inside the European Union. Read more from John Pienaar The row over the migration target comes as the Conservative in-fighting over the EU referendum intensifies. David Cameron and most of his cabinet are campaigning for a vote to stay in the EU, but about half of his MPs support an exit. Conservative Leave campaigner Andrew Bridgen told 5 live's Pienaar's Politics that as many as 50 Tory MPs were ready to back a
summary: David Cameron must accept the failure of the government's manifesto pledge to reduce migration into the UK, two leading Vote Leave MPs have said.