The leader of one of the country's biggest trade unions has threatened to withhold support for the EU constitution "yes" campaign because the government has not done enough for workers' rights.

Amicus general secretary Derek Simpson's comments could undermine the prime minister's claims that the treaty has union backing.

Gordon Brown, meanwhile, urged voters to back the constitution yesterday and said that Tony Blair would remain prime minister for another two years.

The chancellor said the prime minister would lead an all-party campaign for a "yes" vote in a referendum expected in spring 2006.

Asked if he would be prime minister by the time of the referendum, Brown told BBC Radio 4: "Tony Blair is the prime minister and he will remain the prime minister."

Speaking in the Commons yesterday, Blair said the new constitution does not establish a European Union "superstate".

He added that the document "keeps unanimity for the most important decisions".

Michael Howard said the constitution "is bad for democracy, bad for jobs and bad for Britain".

The Conservative leader singled out the inclusion of the Charter of Fundamental Rights as "a case study in government surrender".

Meanwhile a MORI poll has shown an increase from four to nine per cent in support for the UK Independence Party in general election voting intentions in the past month.