Many people with long Covid feel tired, unwell and in pain for lengthy periods after exercise, and researchers say they now know why.

Experts say they have evidence that biological changes are to blame, such as severe muscle damage, mitochondrial problems and the presence of microclots in the body.

“It’s really confirming that there is something inside the body going wrong with the disease,” said Dr Rob Wüst, an author of the study at Vrije Universiteit (Free University) Amsterdam.

Long Covid is thought to affect tens of thousands of people in the UK alone, and many experience a worsening of symptoms for weeks after a single bout of exercise.