By Luke Baker
BRUSSELS, Jan 23 (Reuters) - European officials and diplomats were left scratching their heads after David Cameron's big speech on Wednesday, expressing confusion about how and when the prime minister expects to overhaul Britain's ties to the European Union.
Cameron promised voters an in/out referendum on British membership of the EU if he is re-elected in 2015, saying the referendum would take place by the end of 2017, once Britain has re-negotiated its relationship with the EU.
It may have been what eurosceptics in his Conservative party, and the wider British public, wanted to hear, but it is anything but a straightforward process, and not one that Britain can decide alone.
It needs allies if it wants to distance itself.
Leaving aside the fact that Cameron, down in opinion polls, would need to be re-elected first, the critical question is whether the 26 other EU member states -- or 27 once Croatia joins later this year -- would want to renegotiate the EU treaty, the framework that binds them together.
Fundamental changes to the treaty of the kind that Cameron is hinting at would require support for what is known in EU parlance as a European Convention.
Under EU rules, a simple majority of member states have to be in favour of calling a Convention, so at least 15 countries once Croatia has joined. That is the first hurdle and one Britain might not manage to clear.
If there is a majority in favour of a convention, the EU would begin a long and complex legal and political process involving all member states, the European Parliament, national parliaments and the European Commission.
The convention's recommendations for treaty changes, which have to be unanimous, would then be put to what is known as an intergovernmental conference involving all member states.
Any alterations or renegotiations to the treaty would require unanimity.
It is possible Cameron could try to use a simplified procedure rather than a full treaty overhaul, but this is reserved for changes to treaty provisions for the EU single market.
Given what Cameron wants -- including the repatriation of powers from Brussels to London -- and given that many EU countries are reluctant to simply let Cameron have his way, it is hard to imagine the simplified procedure would be possible.
Instead, Cameron would have to work tirelessly to convince his EU allies that they have something to gain from reopening the treaty, and then convince them again that it is in their interests for Britain to have looser ties to the EU.
French Foreign Minister Laurent Fabius said Britain could not expect Europe to be an "a la carte" menu, or expect to change the rules of membership just for itself.
"Imagine we are a football club. You join the football club, but once you are in, you cannot say 'Let's play rugby'," he said on French radio.
Attitudes to Cameron have hardened since he wielded a veto at a European Union summit late in 2011.
Swedish Foreign Minister Carl Bildt, whose country is a euro outsider like Britain, tweeted: "Flexibility sounds fine, but if you open up to a 28-speed Europe, at the end of the day there is no Europe at all. Just a mess."
Because of the tortuous process that treaty change is, there is a good chance euro zone states will press ahead with closer integration without trying to reopen the treaties.
And even if Cameron got his way, its EU partners would be required to win the consent of their own voters or parliaments for any special deal with Britain -- unlikely to be a big vote winner for them.
Several EU diplomats said it was unclear exactly what Cameron wanted, with some of his speech praising Europe, other parts saying it needed fundamental overhaul and the prime minister ultimately saying he wanted to stay in the EU.
"The speech reminded me of someone picking a daisy: 'Now I love you, now I don't'," said one euro zone diplomat.