David Cameron kept up the pressure on Gordon Brown over the EU Reform Treaty ahead of a key meeting in Lisbon to agree its wording.
The Tory leader issued a fresh challenge to the Prime Minister to honour Labour's manifesto commitment to give the public a say on the constitution.
In a letter to the premier, Mr Cameron urged him to stop his ministers arguing that the latest treaty is less significant than the constitution.
The PM also faced sharp criticism from president of the European Commission Jose Manuel Barroso over Britain's "red lines" on criminal justice matters. The so-called "red lines" would safeguard British sovereignty over areas of policing and judicial affairs as well as foreign and defence policy, social security and labour legislation.
Mr Brown is set to agree the wording of the controversial document this week.
After a meeting with Mr Barroso on Thursday, Mr Brown said he would accept the treaty if Britain's "red lines" were intact. He said the national interest would take precedence and that without opt-outs in key areas like criminal justice and social security he "could not accept" the treaty.
But in a rare rebuke, Mr Barroso said in an interview with The Observer: "Sometimes it appears as a contradiction. Britain, which is always first to ask for global action against terrorism, appears not to be as committed as other members of the EU when it comes to Europe. This surprises me."
In his letter to the PM, Mr Cameron said: "Your 2005 general election manifesto stated without qualification that 'we will put it - the EU Constitution - to the British people in a referendum and campaign whole-heartedly for a "Yes" vote'.
"You will remember telling the BBC the weekend before you became Prime Minister that 'the manifesto is what we put to the public. We've got to honour that manifesto. That is an issue of trust for me with the electorate'.
"This is indeed a matter of trust with the electorate. So, will you now honour the promise you made to the British people to hold a referendum?"
