Protective antibodies in people who have had Covid-19 wane “quite rapidly”, according to researchers.
The number of people who tested positive for antibodies fell by 26 per cent between June and September, a team at the Imperial College London found.
The waning of antibodies means immunity appears to be fading and people are at risk of contracting coronavirus multiple times.
In England, more than 350,000 people have so far taken an antibody test as part of the REACT-2 study.
In the first round of testing at the end of June and beginning of July, around 60 in 1,000 people had detectable antibodies, compared with 44 in 1,000 in the latest round of testing.
"Immunity is waning quite rapidly, we're only three months after our first [round of tests] and we're already showing a 26 per cent decline in antibodies," said Professor Helen Ward.
There was a steeper decline in those over 65 and in those who were asymptomatic compared with those who had symptoms.
English healthcare workers with antibodies remains relatively high with researchers suggesting it may be due to regular exposure to the virus.
Antibodies are a key part of immune defences and stop the virus, with image caption Y-shaped antibodies sticking to the surface of viruses to stop them infecting the body's cells
The meaning of the drop is immunity is still uncertain with other parts of the immune system, including T-cells potentially playing a role in killing infected host cells and calling to other immune cells to help out.
Antibodies do tend to be highly predictive of who is protected, researchers warn.
"We can see the antibodies and we can see them declining and we know antibodies on their own are quite protective,” Professor Wendy Barclay said.
"On the balance of evidence, I would say it would look as if immunity declines away at the same rate as antibodies decline away, and that this is an indication of waning immunity."
There are four other human coronaviruses, which humans can be re-infected with every six to 12 months, causing common cold symptoms.
The research comes as Britain announced 102 new deaths of people with Covid-19, bringing its official total to almost 45,000, the highest toll in Europe.
Official figures show that the number of Covid-19 deaths in the UK rose by 60 per cent in the week of 16 October.