David Cameron has put the call for a referendum on the Lisbon Treaty at the centre of the Conservatives' campaign for the June 4 elections to the European Parliament.

Launching a new campaign website, giveusareferendum.com, the Tory leader accused Gordon Brown of hypocrisy for failing to give the British people the chance to vote on the treaty.

“By any account, the Lisbon Treaty is hugely significant,” he said. “It is, by all accounts, a constitution. That is why we are making this such an important issue at these European elections.”

The Conservatives are expected to make gains after the last elections to the European Parliament in 2004 were dominated by UKIP, which won 16.1 per cent of the vote, and were held when the Tories were still in turmoil.

Robert Kilroy-Silk, the former Labour MP and television presenter, fought a high-profile campaign on behalf of UKIP that year.

In 2004, the Tories received 26.7 per cent of the vote, compared to Labour's 22.6 per cent. An opinion poll suggests that the Tories could improve that figure by at least seven points.

The decision to focus on the Lisbon Treaty reflects the Tories' determination to pick up any voters who are considering deserting the anti-European party.

However, the party will have to make significant advances to pick up many additional MEPs from the 27 it won last time around.

Labour has always argued that a referendum is no longer necessary, as the old constitution was withdrawn after it was defeated in referendums in France and the Netherlands.

Although Britain has now ratified the treaty, it has yet to be ratified by the Czech Republic, Poland or Ireland, where it was rejected in 2008 in the only referendum to be held anywhere in Europe on the treaty.

Mr Cameron again refused to say whether the Conservatives would hold a referendum if they won the next election but the treaty had already been ratified by all the member states.

He simply repeated the previous formula that they would “not let matters rest there”.

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