The number of patients reporting symptoms of Covid-19 weeks after becoming ill could be as high as 20%, an immunologist said, as he warned it could place an additional burden on the NHS.

Professor Danny Altmann, from Imperial College London, said a figure of one in 20 people suffering long Covid, reported in October by King’s College London, is lower than what is now being reported.

“Many people would have 10-20% as their range if you look at the papers on how many people are still reporting significant symptoms several months afterwards,” he told Times Radio.

Prof Altmann is calling for long Covid clinics to be opened in the UK, and recently made the case at a World Health Organisation meeting for mechanisms to be in place for people to be referred to relevant specialists.

He said: “The point I was trying to make was really a call to arms for some really good solid research and mechanisms here, because there’s no point just having the banner above the clinic if we haven’t got the doctors to sit in them.”

Symptoms of long Covid include ongoing fatigue, loss of taste or smell, respiratory and cardiovascular problems and mental health issues.

Prof Altmann warned that there is “no absolute correlation” between severe illness and long Covid, adding: “There’s plenty of people out there who barely know they had it and never had a PCR test or antibody test who are suffering massively still from long Covid.

“It really worries me, the kind of burden I’ve been suggesting is equivalent to our NHS burden of arthritis, which is one of the biggest causes on our NHS time and expenditure.

“It’s a real thing to worry and think about for the future, for patients and healthcare planning.”

However, he expressed optimism as figures showed there were 25,621 patients in hospital in the UK with Covid-19 as of February 9, down from a peak of more than 39,000 in mid-January.

Health Secretary Matt Hancock said the UK could live with coronavirus in the future in the same way as flu, and hoped Covid-19 will become a treatable disease by the end of the year.

Prof Altmann said: “I agree with the ‘by the end of the year’ part, I think the jury’s out on what the future will look like.”

On news of the number of coronavirus patients in hospitals going down, he said: “We can’t easily pick apart how much of that is lockdown, how much is vaccination, but it’s certainly both of those things.

“I am cautiously optimistic that we are winning, finally.”