The European Union must amend its founding treaties to accommodate Britain's renegotiation drive, Foreign Secretary Philip Hammond said on Thursday, warning that London needed a meaty deal to persuade voters to stay in the bloc.

Hammond spoke as Prime Minister David Cameron started a two-day European tour to try to charm other EU leaders into backing his reform drive, something he has promised to complete before giving Britons an EU membership in-out referendum by the end of 2017.

"If our partners do not agree with us, do not work with us to deliver that (reform) package, then we rule nothing out," Hammond told BBC radio.

"The advice we're getting is that we will need treaty change," said Hammond, explaining it was necessary to render changes that Cameron wants irreversible and safe from legal challenge.

Cameron has various demands, the thrust of which is to claw back powers from Brussels to allow Britain to opt out of what he sees as a dangerous shift to greater political integration.

Cameron is due to meet Dutch Prime Minister Mark Rutte in The Hague and French President Francois Hollande in Paris later on Thursday before heading to Warsaw and Berlin on Friday for talks with Polish Prime Minister Ewa Kopacz and German Chancellor Angela Merkel.

Ahead of the talks, French Foreign Minister Laurent Fabius said Paris would listen to Cameron because it wanted Britain to stay in the EU and was ready to consider some reform if it would benefit all member states.

But Fabius cautioned that Cameron's strategy was "very risky," and said his government would not sign up to anything that amounted to the "dismantling" of the EU.

"Britain has more to lose. When I say to investors France will stay in Europe, but Britain isn't sure it will, they listen," Fabius told France Inter radio.

Cameron's government is introducing a law into parliament on Thursday to guarantee the EU referendum will be held by the end of 2017. It also disclosed the question voters will be asked, making it "Yes" to stay in, "No" to leave.

Some EU countries have made clear they have no appetite to re-open the bloc's treaties to suit Britain, which wants to alter them so it can restrict and delay EU migrants' access to its welfare system.

But London has long asserted that the treaties would need to be overhauled anyway as part of an inevitable drive to further integrate the euro zone.

In a potential setback for Cameron, a Franco-German paper seen by Reuters shows the two countries have agreed plans to strengthen cooperation among the 19 countries using the euro currency, without changing existing treaties.

Hammond said he expected some EU states initially to adopt a hardline negotiating position. But he said Cameron's government was confident of securing a good deal.

"What matters is getting it right rather than doing it quickly," Hammond said of the renegotiation. "We're certainly not going to trade substantive reform just for getting it done quickly."