David Cameron started a two-day tour of European capitals Thursday in a bid to secure EU reforms as his government published a law paving the way for a vote on whether Britain should leave.
The British prime minister met his Dutch counterpart Mark Rutte for lunch in The Hague and will have dinner with French President Francois Hollande in Paris before meetings on Friday with Polish premier Ewa Kopacz and German Chancellor Angela Merkel.
Cameron set off as his government confirmed that British voters will be asked: "Should the United Kingdom remain a member of the European Union?" in the referendum, which must be held by the end of 2017.
The question emerged in the EU Referendum Bill, which was formally laid before parliament and will be debated by the House of Commons for the first time on June 9.
The EU referendum is going ahead after Cameron`s centre-right Conservatives won a clear majority at this month`s general election.
No precise date for it has yet been set and Cameron has not ruled out holding it as early as next year.
Europe Minister David Lidington told reporters in London that the mood in the government was to hold it "in one sense the sooner the better" but that this should not come at the expense of getting negotiations right.
Before the vote, Cameron wants to persuade other European leaders of the need for reforms to the EU which he says will require treaty change.
In a sign of the obstacles Cameron could face, French Foreign Minister Laurent Fabius said Thursday that the referendum was "very risky" and "quite dangerous".
Fabius added that you could not "join a football club and decide in the middle of the match we are now going to play rugby".Before Cameron set off, a British official said there were "27 nuts to crack" to secure concessions, while the prime minister himself has said he expects "ups and downs" in the renegotiation process.
The British leader intends to speak to every leader of the 28-nation EU before a summit in Brussels next month which will discuss the proposed British reforms in more detail.
Foreign Secretary Philip Hammond said there would need to be "a substantial package of reform" on Europe.
"If we are not able to deliver on these big areas of concern that the British people have, we will not win the referendum when it comes," he told BBC radio.
Among changes Cameron is seeking are tougher requirements for EU migrants to claim state benefits in Britain. He also wants Britain to be able to opt out of the commitment to "ever closer union".
While he has faced repeated questions about when he will outline a clear programme of demands, Lidington said he had advised Cameron to do no such thing.
"My advice to my boss would be on no account publish a full negotiating position because I don`t think that`s sensible in any sort of negotiation at all to set all that out in advance in writing," he said.
If Cameron can secure the concessions he wants, he will campaign to stay in the EU. Most opinion polls currently suggest voters would also back remaining part of the bloc.