David Cameron said this week that he would guarantee an in/out EU referendum in 2017 if he wins the next general election, and will resign if he cannot deliver on it. This solemn promise confirms his profound ignorance of the way the union works and of the formidable obstacles in the way of his plans to renegotiate the terms of British membership. It also means that a victory for the Conservatives in the next election is likely to lead to Britain's exit from the EU.

In a House of Lords debate on the recent referendum bill, Lord Kerr, a former permanent representative in Brussels, pointed out: "You would be plumb crazy if you seriously thought that the right year to bring to a climax a renegotiation … was the year of a French presidential election and a German federal election." As if President Hollande would expect special concessions to Britain to win him votes; as if the German social democrats or Angela Merkel's CDU would support changes to the free movement of people that is a basic principle of the single market. Polls in France and Germany show public opinion overwhelmingly opposed to a special deal for Britain. Furthermore, repatriation of major powers from Brussels almost certainly requires a treaty change.

Cameron's timetable for achieving this can only have been conceived in cloud cuckoo land. First, he would need a majority of the 28 member states to agree to hold a convention – the last convention took 18 months. The next stage would be an inter-governmental conference – the Maastricht inter-governmental conference took a year. Then a deal would have to be ratified by all member states, several of which would have to hold a referendum of their own. No one can assume that all would vote yes. And have Conservatives forgotten that it took Margaret Thatcher five years of arduous negotiations to secure Britain's budget rebate?

It is true that within Europe there is a general mood for change. But different countries want different changes, and from the start of his premiership Cameron has gratuitously offended his potential allies. He left the European People's Party and isolated British Conservatives from other conservatives in Europe. He has alienated Britain's strongest allies, the Poles, as well as the Romanians, Bulgarians and other central EU countries. To appease Ukip he committed himself to a referendum as part of his negotiation tactics. "How do you convince a room full of people, when you keep your hand on the door handle," asked Herman van Rompuy, the president of the European council."How do you encourage a friend to change, if your eyes are searching for your coat?"

Yes, the EU needs change. But the changes that would benefit Britain can only be achieved if we are active and committed members. The odds are therefore overwhelmingly against achieving a deal by 2017. How could Cameron subsequently recommend a vote for staying in if he comes back from Brussels empty handed? His europhobic party would never let him push back the referendum date, and if he defied them they would enthusiastically kick him out and replace him with a proper europhobe. An anti-European Conservative government would then strongly argue for a no vote, supported by a stridently europhobic press.

The circumstances could not be more different from 1975, when the leadership of all three parties and a majority of the press campaigned for Britain to stay in, and when there was no Ukip.

A referendum would make sense when negotiations are complete and we know what sort of union we should be in or out of. But Cameron's impossible timetable destroys the central pillar of his European strategy. Only those who want us to leave, come what may, can wish for a Conservative majority in 2015.