Will there be a significant rebellion of Labour MPs against a war in Iraq? The conventional wisdom is that there will not, that MPs in general are more sheeplike than ever before.

The assumption is wrong. Back-bench loyalty reached its peak under the Tories in the 1950s and early 1960s. There were two sessions in which not a single Conservative MP defied the whip. Since then, MPs have in general become considerably more rebellious. Nor are British MPs especially cohesive by international standards. Compared with almost all other western European parliaments, the sort of behaviour seen at Westminster can appear almost anarchic.

There were 96 back-bench Labour revolts in the 1997-2001 parliament - fewer than in the recent past, but more than in four other full-length postwar parliaments, including 1945-50. When Labour MPs broke ranks, they did so in large numbers, across a broad range of subjects: lone-parent benefit, disability benefit, the terrorism laws, Lords reform, trial by jury, freedom of information, and national air traffic services. The average scale of these back-bench revolts was greater than in all but three other postwar parliaments - and greater than in every period of Conservative government. They involved a total of 133 Labour backbenchers. These cannot be dismissed as the "usual suspects" - even the West Midlands Serious Crime Squad didn't have that many.

Since the 2001 election, back-bench rebellions have increased, with 66 so far, compared to just 16 in the first session of the 1997 parliament. Labour MPs are now collectively older and more experienced; the back benches are peppered with former ministers and whips, wise to the workings of government.

So what can we expect on Iraq? The numbers signing Alice Mahon's early day motion have already exceeded 130. Many more - including many hard-core loyalists - are distinctly uneasy in private. The government has previously placated critics with concessions. But it is difficult to see what concessions can be offered in this case. A fresh UN mandate would work wonders with some, if not all, of the critics, as might firm independent evidence linking Iraq with dastardly deeds. But neither looks to be forthcoming at present. The omens are not good for the government. If there is a vote - and the rebels may need to force one - this could prove to be the mother of all rebellions. The myths about poodles and robots may then be laid to rest for good.

Philip Cowley is the author of Revolts and Rebellions: parliamentary voting under Blair, published by Politico's