Cabinet ministers are threatening to defy David Cameron by publicly speaking out against his deal with the European Union, which they are warning will fail to cut migration. The Prime Minister was handed an offer on Tuesday by Brussels which critics said contained only "watered-down" pledges. The deal will give EU migrants “gradually increasing access” to benefits after they come to the UK - as opposed to the outright ban Mr Cameron had previously demanded. Despite critics describing the deal as “pathetic” and “insubstantial”, Mr Cameron welcomed the offer by Donald Tusk, the president of the European Council, and said he “sure would” take the deal being offered. Last night Cabinet sources accused Mr Cameron of effectively beginning the campaign for Britain to stay in the EU and warned that they will now start speaking out in favour of a “Brexit”. Downing Street is demanding that all ministers continue to abide by “collective responsibility” and continue to support the Prime Minister’s position until the final deal is done at a Brussels summit in two weeks. Mr Cameron was asked by ministers at a Cabinet meeting on Tuesday morning whether they could now speak out in favour of Brexit. One Cabinet source told The Telegraph: “The Prime Minister has fired the starting gun for the EU campaign and made clear he will be leading the charge to stay in. The assumption is that collective responsibility is over.” Boris Johnson became the most senior Tory to openly criticise Mr Cameron’s deal, warning that there is “much, much more that needs to be done”. Mr Johnson sits in the political Cabinet and so is not governed by collective responsibility. Our Political Editor Peter Dominiczak has more on how the deal unfolded.