In a speech to the London School of Economics on Wednesday (9 January), Mr Murphy said that ratification in the UK should be preceded by a "mature debate" over what is the real nature of the EU rather than what some fear it might be.

He was referring to a letter he received this week which described the EU as "a coup d'etat" and "a Papist conspiracy" which was "similar to the threat from Nazi Germany."

The minister maintained, according to the BBC, that the political parties campaigning against the Lisbon treaty should make clear what Britain's ambitions for Europe are and if they include pulling out of the EU, "they should say so, and say what the alternative is."

The comment was addressed to the opposition conservatives who are traditionally eurosceptic. The party is also fiercely critical of the Labour government for putting the treaty to parliamentary ratification despite previous promises to hold a referendum on similar, but now rejected, European Constitution.

The constitution proposed most of the institutional novelties for the 27-strong bloc which eventually ended up in the new Lisbon Treaty. But it was rejected by French and Dutch voters in 2005, causing a two-year-long impasse among its member states over the future of the union's reform.

While the conservatives argue the content of the Lisbon treaty signed on 13 December is almost identical to the constitution and so should be put to a popular vote, the government claims it is substantially different and so does not require one.

Mr Murphy's speech marks an early start to the Europe debate which is set to feature highly on the UK's political agenda this year.

On Wednesday, meanwhile, Portugal said it would not have a referendum on the treaty.

Prime minister Jose Socrates reportedly backed away from the idea after pressure from both London, Paris and the Slovenian EU presidency.

Slovenian prime minister Janez Jansa on Tuesday that all those deciding on the ratification method should bear in mind "not only the domestic issues, but also pan-European issues," AFP reported.

"Europe is very much interconnected and we have to ask ourselves how events in one country influence events in another country where the situation may be somewhat different," he said.

The Portuguese decision to opt for the parliamentary route means Ireland remains the only country to have a referendum on the Lisbon Treaty, probably in the early summer.