DAVID Cameron has warned public support for Britain's continued membership of the EU is "wafer thin" as he prepares for a series of crucial meetings with leaders in three European capitals to press the case for reform.

The Prime Minister sent the message to EU leaders as he defended his promise to hold an in-or-out referendum on Britain's membership in 2017 if the Tories win the next election.

Mr Cameron admitted previous broken promises over referendums on the EU had eroded support.

He was speaking ahead of a whistlestop tour of European capitals – Paris, Madrid and Berlin – this week, during which he will press the case for a more streamlined EU.

The Conservative leader's trip begins when he holds talks with Spanish Prime Minister Mariano Rajoy.

He will also have a working dinner with French President Francois Hollande before meeting the German Chancellor Angela Merkel.

In an interview with a group of leading European newspapers he called for "a Europe that is more open, that is more competitive, that is more flexible, that thinks more about the cost that it is putting on to its businesses, particularly small businesses".

And in a clear warning that Britain's continued membership could not be guaranteed, he added: "I think the best outcome for Britain is our membership of a reformed European Union.

"We need to recognise that consent for Britain's membership of the European Union, and all the ways that it's changed, has become wafer thin in Britain.

"And politicians, if they do their job properly, have to recognise this fact rather than try to brush it under the carpet."

In opposition Mr Cameron was accused of back-tracking on a pledge to hold a vote on the EU after the then-Labour Government ratified the Lisbon Treaty, which was seen as making the EU more state-like.

The Tories dropped their promise of a vote on the treaty but in January Mr Cameron bowed to pressure from inside his party and announced plans for an in-out referendum.

He hopes to make the case for Britain's continued membership of a reformed EU. His plan, he claimed, would cement Britain's place in the club.

He said: "In British politics, the fact that parties and governments year after year promised referendums, didn't hold referendums when they could have done: that damaged consent for Britain's membership of the European Union and there's no good wishing that away.

"It exists; it's a fact. And the best thing to do when you have a problem is to confront that problem, deal with it.

"And to those people who say to me: 'Ah, but you're creating uncertainty,' the greatest uncertainty would be to have this problem and to wish it didn't exist.

"Much better to have a plan for how we make changes to the European Union, how we make changes to Britain's membership, how we secure Britain's membership of a reformed European Union, and we settle this issue. I have a plan."

The PM will also use his European visits to discuss priorities for the forthcoming G8 summit to be held in the UK in June. He hopes the group of leading nations will agree to promote EU-US trade and step up efforts to tackle tax avoidance.

Mr Cameron is also expected to discuss the deteriorating situation in Syria and call for greater diplomatic pressure to be put on President Bashar al-Assad.