David Cameron's timetable for reform in Europe "impossible" David Cameron was facing new pressure over Europe as Tory Eurosceptics and EU experts cast doubt over his ability to secure changes to European law before a promised referendum on the UK's membership in 2017. The prime minister conceded on Friday that some of his plans aimed at stemming the flow of EU migrants to this country - including banning in-work benefits such as tax credits for four years - will require changes to EU treaties. "There is no doubt this package will require some treaty change, and I'm confident we can achieve that," he said. Under the proposals, jobless migrants would not qualify for unemployment benefits, and those who cannot find a job within six months would have to leave the country. However, Tory MPs and experts on EU treaties said that, even if he was able to negotiate such a package with fellow EU leaders, it would be impossible to complete the process of ratification in the 28 member states in time for a referendum on the amended rules in 2017. Unless the referendum was then delayed beyond 2017, British people would be asked to vote "yes" or "no" to continued membership despite the fact that the renegotiated terms could subsequently be vetoed by any single member state. A new EU treaty has to be approved in every member state, either by its national parliament or in a referendum. The Observer understands that British officials - aware of the potential problem - are already examining possible options under which a referendum could be held before ratification had been completed. Charles Grant, director of the Centre for European Reform, an independent thinktank dedicated to promoting a reform agenda within the EU, said: "The British government will not have got all the treaty changes ratified by 2017." He believes the UK may have to ask for what is known in diplomatic jargon as a "postdated cheque" - an assurance that the changes would be implemented in future when they had all been approved by national governments or in referendums. Bill Cash, the veteran Tory Eurosceptic, said: "It's impossible to finalise a new treaty by 2017. Not only time-wise, but also I do not believe the 28 will sign up to it. Robert Oulds, director of the Thatcherite thinktank, the Bruges Group, headed by Tory peer Norman Tebbit, claimed Cameron was misleading the public. It is definitely not achievable. Cameron is hoodwinking people by saying it can be achieved. Mark Leonard, director of the pan-European thinktank, the European Council on Foreign Relations, agreed that any significant treaty change could not be finalised and agreed by all member states by 2017, meaning there would still be doubt about the central premise of a UK referendum. Labour MEP Richard Corbett, former adviser to president of the European Council Herman Van Rompuy, said: "The idea that you can define a treaty change in 2015, agree it in 2016, and ratify it everywhere in 2017 is just totally unrealistic." Pat McFadden, the shadow minister for Europe, said there was a willingness on the part of other EU countries to help the UK, though it was far from clear what would be agreed, and on what timescale. He added: "It is very important that the negotiations are conducted in a constructive way and that they are not driven by an arbitrary timetable that was set to satisfy the Conservative party rather than for the national interest." Tory MP and former Europe minister David Davis said he believed that meeting a timetable that enabled the prime minister to call a referendum in 2017 would be "difficult but not impossible." To do so, Cameron would have to give the EU a deadline of the middle of 2016 to complete the discussions and reach agreement. A referendum would then have to be held before the ratification process had been completed in the 28 member states.