Boris Johnson could be away from work for up to two months

But he remains under ‘close monitoring’, according to Downing Street, and there are fears it could be weeks or months before he recovers.

The prime minister is said to have ignored doctor’s advice by continuing to work long hours while isolating at 11 Downing Street before being advised to go to hospital on Sunday as his condition failed to improve.

Professor Paul Hunter, an expert at the Norwich Medical School at the University of East Anglia, warned that those that spend time in intensive care will need considerable time to recover.

‘Clearly you will need some time to recover,’ he told the Daily Mail.

‘I would expect most people who were that ill, to need at least a month or possibly two to be sufficiently back and to be able to function’.

Professor Anthony Gordon, an Intensive Care consultant at Imperial College London, also agreed that it could be ‘months’ before the prime minister’s heath returns to normal.

He told the Today programme: ‘When you are seriously ill it does take time to recover. It’s not an all or nothing thing…but it can take many weeks to get back to normal full health, or even months’.

According to the Times, senior figures in Downing Street fully expect that he will need a period of recovery.

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‘One week for every day spent in intensive care is the rule of thumb’, it is claimed.