Long Covid may be triggered by the virus's abnormal blood-clotting complication, researchers say.
Hundreds of thousands of patients have survived the initial infection only to be struck down several months later with fatigue and headaches.
But academics tasked with pinning down the debilitating condition are still baffled over the exact cause.
Now experts at the Royal College of Surgeons in Ireland have moved one step closer to an answer – and say the body's clotting system 'may be involved'.
Hundreds of thousands of patients have survived the disease only to be struck down several months later with fatigue and headaches
Study co-author Professor James O'Donnell said: 'Understanding the root cause of a disease is the first step toward developing effective treatment.'
He added: 'Millions of people are already dealing with the symptoms of long Covid syndrome.
'And more people will develop long Covid as the infections among the unvaccinated continue to occur.'
Up to one in three patients who fall severely ill with coronavirus develop dangerous blood clots, it has been estimated.
The unusual complication — which can affect the lungs and other crucial organs — has also been found in small vessels.
Scientists aren't sure why the virus causes clots but they believe it may be the result of a an immune overreaction called a 'cytokine storm'. Others say the clots may be a byproduct of the way Covid invades the human body.
Professor O'Donnell, a haematologist, added: 'It is imperative we continue to study this condition and develop effective treatments.'
Most coronavirus patients recover from the illness within a fortnight after suffering the tell-tale mild symptoms of a cough and fever.
But a fraction of survivors end up becoming 'long haulers' and are left battling a range of complaints, from muscle pains to headaches.
What is long Covid?
Most coronavirus patients will recover within a fortnight, suffering a fever, cough and losing their sense of smell or taste for several days.
However, the virus can persist for weeks on end in 'long haulers' — the term for patients plagued by lasting complications.
Data from the Covid Symptom Study app, by King's College London and health company Zoe, suggests one in ten people may still have symptoms after three weeks, and some may suffer for months.
Long term symptoms include:
The new research, published in the Journal of Thrombosis and Haemostasis, offers hope of finding a cure for the condition.
Medics examined 50 patients with long Covid symptoms – and compared them against 17 healthy volunteers.
The affected patients had an average age of 50 and were analysed roughly 10 weeks after first getting infected.
Blood samples were taken from all of the participants, allowing the researchers to spot any key differences between the groups.
Inflammation levels among long Covid patients had 'normalised', suggesting the body's natural response to fighting the virus wasn't to blame.
But clotting markers – certain substances known to play a role in coagulation – were still elevated among the affected group.
The researchers said levels were especially high among patients who were hospitalised when they first caught Covid.
Dr Helen Fogarty, lead researcher, said the results suggest 'the clotting system may be involved in the root cause of long Covid'.
Academics also admitted there may be holes in their 'small' study and called for larger trials to firm up the link.
Last month researchers also claimed changes to blood cells caused by coronavirus may explain why so many patients suffer from long Covid.
German experts found the virus alters the size and stiffness of red and white blood cells, making it harder to get oxygen and nutrients around the body.
Their small study of 55 people found that these changes can last for several months, which may explain why many Covid patients become 'long haulers'.
Data last week suggested almost 400,000 Britons have been left battling long Covid for over a year.
Almost 1.46 per cent of people polled by the Office for National Statistics complained of being left battling persistent symptoms.