LONDON (REUTERS) - British Prime Minister David Cameron on Tuesday suffered his first parliamentary rebellion since being re-elected last month as he tried to unite his party on Europe by making concessions on the timing and rules of a planned EU membership referendum.

Cameron, who has promised to try to renegotiate Britain's relationship with the European Union before the referendum, promised he would not hold the vote on May 5 next year, a date Eurosceptic lawmakers in his Conservative Party had objected to, his second U-turn in as many weeks to try to placate them.

He also signalled he would partially agree to their demands to limit his government's ability to influence the Europe debate in the final four weeks of the referendum campaign.

Previously, Cameron had stated he was "open-minded" about holding the vote on May 5 next year, the same day as regional elections in Scotland, Wales and Northern Ireland.

But his spokeswoman said the government had introduced an amendment promising not to hold the vote on that date after listening to lawmakers who said May 5 was too soon to allow for proper debate and that it would be confusing to hold the referendum on the same day as the other elections.

Cameron's tactical shift shows the challenge he faces in keeping his party united on an issue that helped topple his two immediate predecessors, despite his winning a surprise outright majority in a national election just weeks ago.

His about-face came as members of parliament debated a law paving the way for the EU referendum by the end of 2017.

Rebel Conservative lawmakers tried to vote through an amendment forcing Cameron to make even more concessions on the referendum. It was defeated by 288 votes to 97.

But 27 of those who voted in favour were from Cameron's own party, which has 330 lawmakers in total.

The rebels said they were concerned that the rules he was proposing for the referendum would unfairly skew the vote.

REBELLION

The rebellion went ahead despite Minister for Europe David Lidington writing to Conservative lawmakers promising Cameron would spend the next few months trying to find a compromise with them on rules which determine how the government can behave in the last four weeks of the referendum campaign.

Cameron had previously said his government could not remain "neutral" and would want to make its own stance clear in those last few weeks.

Lidington said the government had now agreed to show restraint, not to spend public money on mail shots in the last four weeks, and to alter the rules in a way that would satisfy Eurosceptic concerns.

The setback is Cameron's second in as many weeks. He tripped up over Europe at a summit of the Group of Seven Industrial nations last week, appearing to issue an ultimatum to his own ministers over the EU only to swiftly withdraw it.

He is due to discuss EU reform with Italy's Prime Minister Matteo Renzi on Wednesday and to meet with the leaders of Slovakia, Slovenia, Luxembourg and Ireland this week as well as Martin Schulz, president of the European Parliament.