The flu outbreak sweeping the UK has reached "serious epidemic" levels, the government's chief medical officer has said.

Professor Liam Donaldson said that 300 in every 100,000 Britons now has flu, and he warned the figure may be much higher.

He said there were probably twice as many people suffering from the bug than the official statistics indicated, because of the thousands who are suffering without informing their doctors.

A Department of Health spokeswoman said the official statistics showed there were currently 144 people in every 100,000 suffering from flu.

As the usual gauge of an epidemic was 400 people in every 100,000, she said, the flu crisis was therefore not being officially treated as an epidemic by the government.

But she added: "Professor Donaldson is happy to stick by his version that this is an epidemic.

"He believes it is many more than 144 per 100,000. It is very confusing and it depends on which definition you choose.

"Professor Donaldson has looked at his graph and said it is a serious epidemic."

However, pressure on the UK's intensive care units is expected ease within the next fortnight, despite flu cases reaching their seasonal peak.

Some hospitals, such as Eastbourne District Hospital, which serves a more elderly population, have had to hire refrigerated trailers to serve as makeshift morgues when space in existing facilities ran out.

Christine Hancock, general secretary of the Royal College of Nursing, told the BBC that the problem lay not in a shortage of actual beds, but in hospitals having too few nurses to accompany them.

Intensive care nursing involves having more than one nurse - each fully trained in specialist skills - assigned to each patient at any one time.

Many hospitals have already been forced to cancel planned surgery because no intensive care facilities are available for patients following their operations.

Sir Alan Langlands, chief executive of the NHS, told the BBC he was confident people were getting the right care.

He said: "Anyone who will benefit from intensive care is getting that care from the health service."

The number of empty intensive care beds in the country rose to 31, from 20, on Saturday, a Department of Health spokeswoman said.