JACK Straw yesterday fired the starting pistol to an 18-month referendum campaign on the new European constitution by revealing the vote is likely to take place in early 2006. As he did so, his colleague Denis MacShane, the Europe minister, confirmed Westminster's worst kept secret: the general election was on the cards for next May or June.

The foreign secretary's statement came as Tony Blair joined his fellow EU leaders at the grand Palazzo dei Conservatori in the heart of ancient Rome to put his signature to the new European constitution.

The (pounds) 9m ceremony, at the venue where the original treaty establishing the Common Market was signed 47 years ago, marked the start of a two-year ratification process in which the new treaty will have to be accepted into law by the national parliaments of all the member states.

Britain is likely to be one of the last member states to hold its referendum. Spain will be the first, in February 2005, followed by the Dutch and Portuguese two months later, the French next May and the Poles in the autumn. Most of the other EU members have not made announcements about plebiscites. Technically, if one EU state rejects the constitution it is null and void.

Mr Straw said regarding the timing of the vote: ''No precise date has been accepted and it depends partly on the parliamentary process. It is likely to be in early 2006 for the simple reason that in the autumn of 2005 we have the presidency of the EU and it would be practically almost impossible to be running the presidency with a referendum.''

Mr MacShane was more specific, explaining a referendum bill would be included in the Queen's Speech on November 23. He said: ''That will take up the first two or three months of next year (in parliament). We will then have a general election We then have (from July 2005) the six months of the UK presidency of Europe That takes us into early 2006.''

Michael Ancram, deputy leader of the Tories, who oppose the treaty, said: ''It is rubbish to suggest we cannot hold a referendum during our presidency. This is clearly a delaying tactic. After we win the election, we will hold a referendum by next October.''

Sir Menzies Campbell, the LibDem deputy leader, welcomed the treaty signing, saying: ''It is in the interests of the people of the UK and we have nothing to fear from it.''

The ceremony took place against a backdrop of the continuing crisis over the appointment of a new European Commission.

Jose Manuel Barroso, incoming EU president, said he was withdrawing his team of commissioners in the face of opposition from MEPs, who were not prepared to accept Rocco Buttiglione as justice commissioner after he branded homosexuality a sin. Last night, Mr Barroso signalled he was likely to drop some nominees to satisfy critics.

Silvio Berlusconi, the Italian prime minister, said that Mr Buttiglione would remain a minister in his government and it was likely that another candidate would be found to take his place at the EU.