Calls for Euro referendum after MEP claims treaty 'crosses Labour's red lines'

Gary Titley, the party's leader in the European Parliament, conceded the UK had not opted out of the controversial Charter of Fundamental Rights.

His comments will be hugely embarrassing to the Prime Minister, who has repeatedly insisted the charter would be blocked.

Under the charter, to be ratified as part of the revived constitution, Brussels could impose laws on Britain on issues including immigration and crime, and increase trade union power.

Mr Titley's comments will ratchet up the pressure on Mr Brown to honour Labour's election promise to hold a referendum on the power grab.

Ministers have resisted calls for a vote claiming the treaty - which surrenders a raft of British powers - was "substantially different" to the constitution.

But confronted with mounting evidence the two documents were virtually identical, they backtracked and said a vote was unnecessary because Britain would keep control of its "red lines" - covering policing and courts, human and social rights, foreign policy and taxes.

Mr Titley's inflammatory comments were last night set to undermine his leader in Westminster.

In a debate with union leaders, who are pushing Mr Brown to hold a referendum, he said: "The RMT union is arguing that the UK has opted out of the Charter of Fundamental Rights. This is wrong."

In a damning insult to the millions of Britons demanding a referendum, he added: "Most people aren't interested in the processes (of the treaty). They want to know what Europe does, not how it does it."