Blood clots in the brain or the lungs might explain some common symptoms of "long Covid", including brain fog and fatigue, a UK study suggests.

In the study, of 1,837 people admitted to hospital because of Covid, researchers say two blood proteins point to clots being one cause.

It is thought that 16% of such patients have trouble thinking, concentrating or remembering for at least six months.

Long Covid can also develop after milder infections.

But the research team, from the universities of Oxford and Leicester, stress:

• Their findings are relevant only to patients admitted to hospital
• They are "the first piece of the jigsaw" but further research is needed before they can propose or test any potential treatments
• They tracked cognitive problems at six and 12 months only and through tests and questionnaires, which may "lack sensitivity"

Identifying predictors and possible mechanisms was "a key step" in understanding post-Covid brain fog, study author Prof Paul Harrison, from the University of Oxford, said.

But there may still be many different causes of long Covid.

Leicester's professor of respiratory medicine, Chris Brightling, said: "It's a combination of someone's health before, the acute event itself and what happens afterwards that lead on to physical and mental health consequences."

University lecturer Dr Simon Retford, from Lancashire, spent two weeks in a coma after catching Covid in October 2020, with his family told to expect the worst.

Now, he is 60-70% back to where he was before but still has problems concentrating and with short-term memory loss and losing his train of thought.

"I took on a course-leader role last May and I was like a really slow computer which struggled to get going," he says.

Dr Retford used to work for the police but would be unable to do so now.

"If I overdo it now, I feel so fatigued," he says.

But although he may never completely recover, Dr Retford is determined to remain positive.

"I'm still here and thousands aren't," he says.

The Post-hospitalisation Covid-19 study (PHosp-Covid), in Nature Medicine, blames higher levels of the protein fibrinogen and protein fragment D-dimer for brain fog.

Study author Dr Max Taquet, from Oxford, said: "Both fibrinogen and D-dimer are involved in blood clotting and so the results support the hypothesis that blood clots are a cause of post-Covid cognitive problems.

"Fibrinogen may be directly acting on the brain and its blood vessels, whereas D-dimer often reflects blood clots in the lungs and the problems in the brain might be due to lack of oxygen."

Those with high D-dimer levels also:

• complained of extreme tiredness and being short of breath
• tended to have difficulty holding down a job

A US study found similar results.