For Gordon Brown, a row over Europe was always inevitable. British Prime Ministers, right or left, treat the grisly affair as a rite of passage: it goes with the job. Vigilant Eurosceptics lurk at every turn, ready to denounce the latest "sell-out to Brussels". Even a Labour leader who has been cool towards the European project - hence the absence of euros from our pockets - knew his turn was coming.

Brown could probably have counted on David Cameron to adopt the Tory default position sooner or later. Poor polls and fear of a snap General Election have seen to that. The Prime Minister could have guessed, too, that the Sun, Mail and Telegraph would begin to rumble at the word "treaty", his good press would turn bad and the demand for a referendum would blur, instantly, into a campaign to save the nation.

Whether Brown also guessed that perhaps 120 of his own MPs want a plebiscite, according to Glasgow South West MP Ian Davidson, is another matter. And did he realise, when agreeing to address the TUC in Brighton, that he would face ferocious demands for a national vote from the GMB, RMT and others? The minor issue of appearing to reject one of his own manifesto promises begins to seem like small beer.

There are some strange bedfellows here. Stranger still is the government's continuing insistence that it is all a great deal of fuss over nothing. True, Labour promised the country a referendum on the proposed European constitution, but that, it says, became superfluous when France and the Netherlands rejected the idea in 2005. The reform treaty thrashed out by Tony Blair in June this year is in no sense "constitutional", says the government, and bears no resemblance to its predecessor. Therefore, no vote is needed.

Technically, as it happens, Downing Street is correct. The reform treaty amends two previous treaties. It does not purport to be a draft European constitution. By convention, a mere treaty does not require a referendum. Brown has not - technically - breached his manifesto commitment.

Others, far from the Eurosceptic camp, have taken a different view. There is quite a list. According to the Open Europe think tank, it includes Germany, Spain, Ireland, the Czech Republic, Finland, Denmark, Austria, Belgium, Italy, Lithuania, Luxembourg, Slovenia, the European Parliament and the European Commission. Each has said essentially the same thing: the new treaty keeps the "defeated" constitution intact.

Two brief quotes give a flavour of non-British attitudes. Angela Merkel, Chancellor of Germany: "The substance of the constitution is preserved. That is a fact." Bertie Ahern, Taoiseach: "Ninety per cent of it is still there these changes haven't made a dramatic change to the substance of what was agreed back in 2004."

Perhaps, though, this is all just spin from leaders refusing to admit defeat? Perhaps even the 10% to which Ahern alludes has made all the difference and allowed the British government to claim, as it persists in claiming, that the new document is essentially different from its predecessor?

It's a tough case to make, even when you realise, with a sinking heart, that the language of the new draft treaty is close to unreadable. (No accident, according to Karel de Gucht, the Belgian Foreign Minister: "The constitution aimed to be clear, whereas this treaty had to be unclear. It is a success.") Unpick the provisions, however, and you realise that most - 240 of 250, according to one analysis - of the proposals in the "new" document preserve the intent of the rejected constitution.

And what might those be? If you are an arch-federalist or a conviction sceptic, they amount to a foundation document for a true European state. If you are of a less excitable frame of mind, you could argue, merely, that a union of 27 nations would become unworkable without the proposals contained in either text. What you cannot say, as the government attempts to say, is that the proposals alter nothing of importance.

When MPs return in October, they will have to decide, in short order, whether a permanent European president should replace the existing part-time, rotating presidency. MPs will have to vote for or against a European foreign minister and diplomatic service. Britain's parliamentarians will have to decide whether they fancy a "self-amending treaty" that can be altered incrementally, without a need for full inter-governmental conferences.

Then Westminster will have to decide if it can swallow a cut in the number of commissioners after 2014, meaning that one-third of member states would not be represented within the European Commission at any given time. That would include Britain.

When parliament returns, MPs will also have to see their way to accepting the extension of qualified majority voting to 61 more "areas". In other words, over a swathe of matters - including aspects of foreign policy - the veto would be abolished. If Open Europe's research is to be trusted, Britain's power to block European legislation would, meanwhile, be reduced, thanks to changes in the voting system, by 30%.

The treaty further provides for the European Parliament to obtain "co-decision" powers with heads of government in 40 new areas, including the common agricultural policy. It would acquire vetoes of its own. A European public prosecutor, capable of ordering prosecutions in Britain, is also on the cards. Then, in the jargon, there is the proposal that the EU should have "a single legal personality".

This may be the most important single decision facing MPs. Europe collectively can already sign treaties in matters such as trade. How about defence, foreign policy, crime? A European army, police force, or a European seat on the UN Security Council? Such ambitions are easy to deny, but all fall within the scope of the new treaty. Most of Europe's governments seem content that this is so, and there is a case to be made for each of them.

How is a 27-member union supposed to operate otherwise? Brown's ministers seem determined to repeat that their "red line" issues - opt-outs concerning justice, home affairs, social security, tax, foreign policy and the Charter of Fundamental Rights - have been preserved. The truth is that changes in other areas could have a decisive impact, in time, even on these.

The unions, meanwhile, cannot see why 26 countries should allow a greater right to industrial action while we "opt out". Brown's back-benchers want to hold him to a manifesto commitment while keeping corporate Europe in check. Keith Vaz, former Europe Minister, and David Blunkett, former Home Secretary, demand a referendum soon, if only to lance the boil once and for all.

Polls suggest, however, that the government would fail in the Yes campaign to which it would be obliged, in honour, to subscribe: Blair signed up for the treaty, after all. Brown doesn't need defeat so early in his premiership. Perhaps, then, he hopes Westminster will do his work for him: that a defeat aided by his own back-benchers, though embarrassing, will rid him of the treaty and spare him a referendum. If a plebiscite is the core demand, however, his choices are few.

As they should be, I think. New Labour has been dodging and weaving around the European issue for a decade. It is time, surely, to face these questions once and for all. What kind of Europe? And what part, if any, does Britain wish to play?