A Covid expert has poured scorn on government claims that omicron infections have reached 200,000 a day, branding the figure implausible.

       Ministers have failed to explain how the estimate – announced by health secretary Sajid Javid one day before a huge Tory revolt against the Plan B restrictions – was calculated.

       Now David Spiegelhalter, a statistician at the University of Cambridge, has attacked the sudden use of the figure – suggesting the real daily tally was 45,000 new cases on Monday.

       “It’s bit naughty to give this number without having a justification behind it,” Prof Spiegelhalter said.

       “It’s not based on the published modelling that came out from UK Health Security Agency a few days ago.

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       “If you put that through their model it comes to 45,000 infections yesterday – which is high, but does seem more plausible.”

       The professor of the public understanding of risk, and a regular commentator on pandemic modelling, added: “From what has been made public, I can’t see how you can derive this number.”

       The latest estimate is that omicron cases are doubling every 2-3 days – the reason why Plan B curbs are coming in – which implies 1.2 million new infections by Christmas Day.

       But Prof Spiegelhalter suggested a tailing off is inevitable, telling BBC Radio 4: “Whatever the line is, it’s got to curve over sometime – because we are not going to get up to these vast numbers of millions of people getting it every day.”

       After the shock of the 200,000 figure put out by Mr Javid, his officials were unable to explain how it had been calculated, other than to say it is based on “modelling”.

       Omicron is believed to make up around one-fifth of all Covid cases, but as many as 44 per cent in London, the UK hotspot for the new variant.

       It is expected to become the dominant strain in the capital by the end of Wednesday at the latest, Mr Javid told MPs on Monday.

       However, the government is struggling to persuade its own MPs to back curbs already introduced – let alone more severe measures many are calling for before Christmas.

       Chris Whitty, the chief medical officer, warned the cabinet to expect a “significant increase” in hospitalisation, while hospital leaders warn parts of the NHS are already “beyond full stretch”.

       But around 70 Tory MPs have signalled they will vote against Covid passes for crowded venues – with less opposition against the extension of mask-wearing to cinemas and theatres.

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       It is not the first time Prof Spiegelhalter has criticised the government’s use of Covid statistics as the pandemic is progressed.

       Last May, he attacked the announcement of “unreliable” daily test totals, as “number theatre”, when the public was crying out for “proper detail”.