Scientists say they have detected irregularities in the blood of long Covid patients that could one day pave the way for a test for the condition.
Imperial College London researchers found a pattern of rogue antibodies in the blood of a small number of people with long Covid.
They hope it could lead to a simple blood test within six to 18 months.
Dr Elaine Maxwell, from the National Institute of Health Research, said the early findings were "exciting".
She said there could be "a number of different things happening after a Covid-19 infection" and an autoimmune response "has been one of the suspected mechanisms".
But she warned that long Covid was a "complex condition", saying it was important to continue to research the other causal factors so all different types of post-Covid syndrome could be diagnosed and treated.
Long Covid is not yet fully understood and can cover a range of symptoms lasting long after initial infection including fatigue, breathlessness, headaches and muscle pain.
There are currently no tests to diagnose long Covid.
Prof Danny Altmann, who is leading the research team at Imperial, said he believes the work will lead to a test which could be done in a doctor's surgery.
But he said he was concerned the UK government's plans to "live with" Covid-19 could be stoking the next wave of cases of the condition.
The government plans to end most coronavirus restrictions in England in a week's time, on 19 July.
But amid rising cases Prof Altmann warned that it is not yet known if vaccinations will protect people from long-term symptoms.
"If we're heading into a phase of 100,000 cases per day, and, we're saying that 10-20% of all infections can result in long Covid, I can see no certainty that we're not brewing those long Covid cases despite having a vaccinated population," he said.
In the pilot study, researchers compared the blood of dozens of people and found what are called autoantibodies that were not present in people who recovered quickly, or those who have not had Covid-19.
Normally, human immune systems create antibodies to fight disease. But sometimes the body turns on itself - creating the autoantibodies that attack healthy cells.
Prof Altmann believes these autoantibodies may be one of the things causing long Covid symptoms.
He said it is also possible that some people still have the virus "persisting" in their bodies, while others may have other problems with their immune systems.
The research is still at an initial stage, meaning the research was done on a small sample size which can be scaled up later.
Prof Altmann warned that the findings cannot yet be described as a breakthrough, but said they were "a very exciting advancement".
An estimated 962,000 people in the UK had long Covid in the four weeks up to 4 June, according to data from the Office for National Statistics.
And around 385,000 people in the UK are estimated to have been suffering with symptoms for more than a year.
Long Covid can affect people of all ages, including children and people who were previously fit and healthy, as well as those who only experienced mild coronavirus symptoms at first.
'I had long Covid, and then caught Covid again'
I am 44 years old - but in my body, I'm like someone twice my age.
I caught Covid in March last year. Having long Covid means I wake up in pain and go to bed with pain. I have vertigo, migraines and blurred vision.
After 11 hard months, I began to feel exactly as I had almost a year previously.
I went for a test and that evening I got an email saying I had Covid again.
Read Lucy's story here.
In May, the UK government promised to set up 89 long Covid clinics to help patients with specialist care.
The guidelines say these clinics should be doctor-led and should accept patients regardless of whether they had needed hospital treatment for Covid-19.
However, BBC Panorama found some clinics are not staffed by doctors - and four of them only see patients who were originally hospitalised with Covid.
Ten clinics had patients who had been waiting more than 100 days and the longest wait was more than six months, at 191 days.
In response, NHS England said it has "invested more than £134 million" in long Covid services and is setting up 15 paediatric hubs.
Neither Scotland nor Wales have specialised clinics. Both told the BBC they were strengthening existing services. Scotland said it was investing in £2.5m in research.
Northern Ireland's first long Covid clinics will open in October.
A spokesperson from the Department of Health said: "The government rapidly provided specialist care for acutely ill Covid-19 patients at the start of the pandemic and we've matched that speed and scale in our support for people with long Covid."
They said they had backed scientists with over £50m for research to better understand the long-term effects.
Panorama's Long Covid: Will I Ever Get Better? will be on BBC One at 19:35 BST on Monday 12 July and on BBC iPlayer (UK only)