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The latest Tory call for an EU referendum - delivered to the prime minister last night by backbench awkward squaddie John Baron and an anonymous accomplice - adds new data to the eurosceptic numbers game among Conservative MPs.

The backbench motion calling for a referendum, last October, was supported by 81 Tory rebels; now "100 plus" Conservative MPs, including some ministerial aides, have backed Mr Baron's call for the government to legislate for a referendum after the next election - and Mr Baron adds that he could have collected more signatures, but wanted to deliver his letter in good time.

In short, he thinks he has the support of practically every Conservative MP outside the government, and he expects that fact to weigh heavily on the PM.

His proposal is to commit the next government to holding a referendum, and put whoever is in charge in the position of having to repeal a law, if they want to avoid the referendum. More than that, party leaders would certainly face questions about whether they would go ahead with the referendum, in the course of the next election campaign.

One key detail is not set out: the question which would be put to the public at the referendum. Mr Baron argues that, two years from now, the EU is likely to have been transformed by the euro crisis, and it is impossible to know what kind of organisation it will have become. So he proposes that the referendum question should be on the "nature of our relationship with the EU".

There's plenty of euro-sceptic activity in the undergrowth at the moment - with cross party discussions about the response, if there is, for example, a collapse of confidence in another euro-zone country, or the expulsion of one of the member states from the single currency.

Next week, David Cameron will report back to MPs on the outcome of the latest EU summit. He can expect a rough ride. And that will be just another step in what promises to be an unrelenting campaign.