The NHS is "more scared than we have ever been" about the risks of a heavy flu season this winter, amid fears the vaccination may fail to protect the elderly.

Sir Malcolm Grant, chairman of NHS England, last night said he fears hospitals will be "inundated" with cases, despite attempts to bolster services.

His warnings came as the country’s Chief Medical Officer told the Daily Telegraph that she fears the jab may not protect the elderly this winter, but said it is still “our best hope” to stave off an NHS crisis.

Dame Sally, England’s most senior doctor today urged 21 million people eligible for free vaccinations - including young children, pensioners, and health workers - to take up the jabs.

However, last year’s jab had zero effectiveness among over 65s, an official evaluation reveals, while protecting two in three children.

This year’s programme will push further on trying to protect those who come into contact with pensioners, in case the vaccination fails as badly this year, with fears flu rates could be far higher.

In a letter sent to all NHS workers, staff are told to “do their duty” and get vaccinated - with those who opt out told they will have to explain themselves.