* UK PM party's came third in European elections
* The Eurosceptic UKIP came first
* Cameron under pressure to toughen EU policy in response
By Andrew Osborn
LONDON, May 27 (Reuters) - British Prime Minister David Cameron on Tuesday came under pressure from business leaders and donors to spell out in detail which powers he wants to claw back from the European Union after his Conservative party came third in European elections.
The calls put Cameron on the defensive ahead of an EU summit in Brussels later on Tuesday at which he is expected to push for deeper EU reform and to start informal talks about who gets a clutch of top EU jobs.
Cameron has pledged to try to renegotiate Britain's EU relationship if re-elected next year before giving Britons an in/out membership referendum in 2017. He is now more under pressure over Europe after his party was pushed into third behind the Eurosceptic UK Independence Party and the opposition Labour party,
Some of his own Eurosceptic lawmakers and supporters are frustrated that he has not yet laid out in detail which powers he wants to repatriate and has failed to identify "red lines" or areas where he would not back down in any negotiation.
On Tuesday, in a letter to The Daily Telegraph newspaper, prominent business leaders and party donors called on him and other party leaders to explain to voters what kind of a vision they had for Britain in the EU.
Such action was necessary, they said, after a majority of British voters in the elections to the European Parliament backed parties which favour giving Britons an in/out referendum.
"Political leaders should recognise the power of that vote and set about explaining how they will deliver EU reform, a better deal for Britain and an in/out referendum," the letter, signed by 44 individuals, including members of the pro-reform Business for Britain lobby group, said.
"The Conservatives and the Green Party have begun to sketch out their vision for the EU, but the other parties have only paid lip-service to reform without providing any detail. Vague promises will only increase voter apathy and the numbers using their vote as (a) protest."
Labour, which came second, only favours a referendum if there is a new major transfer of power to the EU, something it deems unlikely. The Liberal Democrats, who came fifth, agree.
'CARDS ON TABLE'
Cameron's EU reform policy is an important part of his campaign to try to win a national election in May next year since he is arguing that his is the only party with a realistic chance of coming first that is promising a referendum.
Any re-negotiation of Britain's EU ties would begin after the vote in May 2015 if and when Cameron is re-elected.
Cameron has said he wants to make the EU's policies more business-friendly, give national parliaments more power, and restrict unemployed EU nationals' access to other countries' welfare and social security systems.
His ideas have so far won only limited backing from fellow EU leaders such as powerful German Chancellor Angela Merkel.
On Monday, Cameron rejected calls to bring forward the date of the promised British referendum by a year.
On Tuesday, his spokeswoman defended his EU stance, saying the prime minister had set out his vision for a future relationship with the bloc but that he didn't want to reveal more because that would undermine his negotiating position.
"He has been clear in the vision he set out ... on the areas where he would like to see reform," she told reporters.
"More powers flowing back from Brussels, less red tape, more free trade agreements, less interference from Brussels and the ECHR (the non-EU European Court of Human Rights) in police and justice systems.
"But, the point that he made then is that we also need to recognise this is a negotiation and it wouldn't be smart to lay all Britain's cards on the table at the outset."
(Additional reporting by William James; Editing by Jeremy Gaunt)
((andrew.osborn@thomsonreuters.com)(+442072195389)(Reuters Messaging: andrew.osborn.thomsonreuters.com@thomsonreuters.net))
Keywords: EU ELECTION/BRITAIN
