The British Conservative Party has unveiled its policy programme for the upcoming election on May 5th with a pledge to win a "No" vote on the EU treaty.
Prime Minister Tony Blair, is expected to secure a third term, albeit with a reduced parliamentary majority.
However, opinion polls show the Conservatives just a few points behind in what is shaping into the first real contest since 1992 as anger over the Iraq war erodes support for Mr Blair.
"We oppose the EU constitution and would give the British people the chance to reject its provisions in a referendum within six months of the general election," said the Conservative policy manifesto.
Were the Conservatives to defy the odds and oust Labour, the consequences for Europe would be far-reaching.
Such a result would hand the reins of the EU to a eurosceptic party that will campaign to reject the bloc's constitution during Britain's six-month presidency from July.
All 25 EU members must approve the treaty.
The Conservatives also want to renegotiate central planks of London's relationship with Brussels, including existing treaties on employment laws and fishing that would require the unlikely agreement of all member states.
Growing support for the Conservatives' position on the treaty prompted Mr Blair to perform a huge U-turn last year when he pledged to hold a referendum on the constitution in 2006.
Polls show most Britons oppose the EU constitution and a "No" vote could force Mr Blair out of office, analysts say.