Cameron: 'PM betraying voters over referendum'

Gordon Brown is under mounting pressure over Europe as he prepares to attend his first European Union summit as Prime Minister this week.

David Cameron warned Mr Brown that he would betray the voters' trust unless he promised a referendum on a new EU treaty, due to be agreed by EU leaders in Lisbon on Thursday and Friday.

A senior Labour MP warned that the special opt-outs won by Britain could prove worthless, while the European Commission said they would hamper the fight against terrorism.

In a letter to the Prime Minister, the Tory leader recalled that Labour had pledged a referendum on the proposed EU constitution, the forerunner to the treaty, at the 2005 election.

He told Mr Brown: "This is indeed a matter of trust with the electorate. So, will you now honour the promise you made ... to hold a referendum?"

Michael Connarty, the Labour chairman of the Commons European Scrutiny Committee, said that if Britain did not sign up to common EU policies on justice and home affairs within five years, it could be excluded from all EU co-operation on such matters.

He said: "Britain's 'red lines' will be crossed in five years. They have given us five years to get into line. The 'red lines' will be basically rubbed out."

Jose Manuel Barroso, the European Commission president, told The Observer: "Britain, which is always first to ask for global action against terrorism, appears not to be as committed as other members when it comes to Europe. To fight international terrorism and crime, we will need more, not less integration."

Jim Murphy, the minister for Europe, insisted the opt-out on home affairs was not time-limited. "If we don't achieve the abandonment of the constitution ... and if we don't achieve all of our red lines and our unique deal, then we wouldn't ratify this treaty."