Michael Howard has pledged to renegotiate any ratified EU constitution, as reports suggest that the government is prepared to sign a new treaty.

The Conservative leader's attack comes amid concerns that the forthcoming EU parliamentary elections could become an unofficial referendum on the document.

Speaking yesterday, Howard argued that the constitution amounted to the largest constitutional change since the 17th century.

If the Conservatives win the next election, he said he would "start negotiating about it to change those arrangements".

However, one minister told the FT that, if the government were able to ratify the treaty before the next general election, the Tories would be on the defensive on this issue throughout the campaign.

"We would be able to portray the Tory pledge to amend the constitution as little more than a promise to quit the EU altogether," he said.