For once, the prime minister has plenty of reasons to be gloomy. What with the nation’s mortgage lenders at risk of catching a US cold and the nation’s livestock in danger of contracting Foot and Mouth, Gordon Brown has every excuse to wear a frown. But as if he didn’t have enough to worry about, the PM’s been fretting about another contagion entirely – EU referendumitis.

You see, the problem is this disease really seems to be catching. The demands for a referendum on the EU treaty started, naturally enough, with the Conservatives. Then some of the usual suspects on the Labour backbenches caught the bug.

But what’s really worrying Number 10 and the whips is the number of ex-ministers and formerly loyal Brownites who now appear to have been infected: the likes of Gisela Stuart, Keith Vaz, – former ministers both – and Ian Davidson, the Glasgow South West MP who’s long been a Brown cheerleader. Marginal MPs, under pressure from their local newspapers, are also agitating behind the scenes.

One senior source in the whips’ office told me there were now serious concerns that, so great is the opposition, Brown might get defeated in Parliament when he tries to ratify the treaty. Even allowing for a bit of exaggeration from the whips (hoping presumably to scare MPs into submission) ministers and mandarins alike insist that, despite the other troubles furrowing Brown’s brow, he’s expending a fair amount of energy angsting about Europe.

And let’s face it, he should be worried about the political ramifications of refusing to hold a referendum. One of his first prime ministerial promises – oft-repeated since then – was to give people more say over political decisions. What’s the point of holding a citizens’ jury on every subject under the sun and then refusing to give the public a say on one of the issues they most want to vote on?

When you think how readily Brown has embraced a Conservative agenda on everything from immigration to gun crime (oh, and don’t forget that meeting with Maggie T), it’s somewhat surprising that he’s resisting a rightwards shift on Europe.

There’s been a lot of to-ing and fro-ing on the subject while the rest of Parliament was holidaying in the last few weeks. The PM is apparently acutely aware he’s got an uphill struggle to stop a Labour rebellion gathering momentum. He and the foreign secretary David Miliband are said to be planning a series of ‘charm offensives’ to try and wrestle the doubters back in line.

And there are plans to promise plenty of time for MPs to scrutinise the treaty as it passes through Parliament for ratification. Ministers hope that when backbenchers have time to look at the small print of the treaty they’ll realise it’s not as scary as the Tories say.

But if the government’s serious about setting time aside to ratify the document, that presents a problem. I fail to see how Brown can hold an early election (by which I mean this autumn or spring 2008) and get the treaty agreed by Parliament.

Once the EU has agreed the final text, the bill to ratify the treaty wouldn’t be ready to start its passage until December. It will take between four and six months to get it through Parliament. So when you take into account Christmas and Easter recess, Brown could only get the bill through by May, and even that would be a rush.

It would just about be possible to ratify the treaty and hold an election in late June, but even then the risk is the campaign would be dominated by Europe – something that would play into the Tories’ hands.

Labour rebels fear the prime minister could shoot their fox by holding an early election and putting a line in the party’s manifesto about Parliament – not the public – having its democratic say on the treaty. But that wouldn’t negate the issue during an election campaign. If anything, it would inflame Labour, and certainly the Tories, even more.

I remain convinced Brown wants to get Europe out of the way and off the agenda before he goes to the country. For that reason, and that reason alone, I’m enrolling at the growing school of thought that spring 2009 is the most likely date for a general election.

But as Brown must have brooded in the last few months, as he watched one disease after another sweep the country, who knows what new sickness may be incubating to scupper his carefully laid plans. Avian flu anyone?

Cathy Newman is political correspondent of Channel 4 News