The Foreign Secretary made clear that the Government would try to persuade a sceptical public to endorse the treaty by warning them of the severe consequences of a "no" vote. He said rejecting the treaty would be a "risk" and leave the country in "unknown territory".
Opening a Commons second reading debate on the Bill to ratify the constitution, Mr Straw said Britain would be left "weak and isolated" in Europe by a "no" vote. "We would have no option but to go cap in hand to Brussels to ask our partners to start all over again, reopening negotiations in which we have secured such a good result.
"If we got any deal at all, it would be a worse and not a better deal, negotiated from a position of weakness and not of strength. The long-term effect would be Britain falling into a semi-detached position in Europe while others went ahead without us. We'd be left without influence, out on the margins with no say in Europe's future direction."
Mr Straw accused the Tory opposition of using the referendum as a "Trojan horse" to renegotiate existing EU treaties as well as the new one implementing the constitution. He said their goal was "pure fantasy" and "literally undeliverable" as it would require the agreement of all 24 other member states. "I am confident this patriotic case and patriotic argument for Britain in Europe will win against the narrow pessimistic isolationism of the anti-Europeans," he said.
But Michael Ancram, the shadow Foreign Secretary, said the Tories would oppose the Bill because they believed the treaty and the constitution were separate issues that should be dealt with by separate legislation. If a Tory government were elected in a May general election, it would hold a referendum before the end of September, he pledged. The choice, he said, was between "a Government hell-bent on giving Brussels more control over our lives and a Conservative Party that will reverse the conveyor belt".
The debate highlighted the sharp differences inside the two main parties on Europe. Kenneth Clarke, the former Tory Chancellor, accused Labour of a weak "U-turn" by agreeing to a referendum, saying the decision on ratifying the treaty should have been left to Parliament. But he refused to join Tories in opposing the Bill.
Mr Clarke warned the referendum would be a "lottery" as other plebiscites had been. "It will be very difficult to get large numbers of people to vote for it, on it at all. Most people will continue to say, as they do now, that they are not quite sure what it's all about and what the treaty is actually going to do," he said.
Labour MPs who oppose the constitution warned that it would hand substantial powers to the EU over policies such as public services, trade and asylum. Ian Davidson, MP for Glasgow Pollok, said: "Signing up to this constitution means handing 63 new powers to the EU and Labour voters should say `no'. The constitution would mean Thatcherite economics, a militarised EU and the centralisation of power. There should be no new powers to Brussels until the EU is reformed."
Sir Menzies Campbell, foreign affairs spokesman for the Liberal Democrats, criticised the Government for not making the European case stronger. "I think the Government could have done more but I still believe the referendum campaign can be won so long as there is full engagement," he said.
Robin Cook, the former Labour foreign secretary, criticised the "22-carat nonsense" voiced by opponents of the constitution. He denied that it was an escalator to greater power for Brussels.