Its findings showed that by June, after the first wave of the pandemic, just 6 per cent of the population had developed antibodies.
Three months later, however, the study showed that had dropped to 4.4 per cent - with most of the decline happening within just six weeks.
The sharpest fall was in those most in need of protection, with antibody levels among over-75s reducing by close to 40 per cent between June and September.
Scientists said the findings showed Britain was "miles off" achieving herd immunity, which they said might never be reached without a vaccine.
However, the research did not examine the role of other forms of immunity.
Some scientists believe the part played by T cells - a type of white blood cell that helps the immune system fight viruses and which is linked with prior infections with common colds - could be more crucial in fighting the virus.
Scientists analysed home fingerprick samples from hundreds of thousands of adults, to establish "detectable antibody levels" over three months, and found levels fell by 26.5 per cent overall.
The largest fall was among those most vulnerable to serious illness from Covid-19.
Among those aged 75 and above, antibody levels fell by 39 per cent, while a drop of just 15 per cent was seen in those aged between 18 and 24.
Researchers stressed that it was not yet known what level of antibody response was required to protect against reinfection, meaning it was possible that even low levels of antibodies offered some protection.
However, they said the findings suggested a significant rapid decline in immunity - raising the prospect that those infected could suffer repeatedly from infections in further waves.
The findings suggested that even if a successful vaccine was developed, it might have to be administered as often as every six months, increasing the scale of the challenge ahead.
However, researchers said vaccines could prove more powerful than natural immunity.
Professor Helen Ward, the lead researcher and an Imperial public health expert, said: "I think what we are showing is that there is a really big challenge to that, which is immunity is waning quite rapidly.
"After three months, we've already shown a 26 per cent decline in antibodies. When you think that 95 out of 100 people are unlikely to be immune, and therefore likely to be susceptible, then we are a long, long way, from anything resembling a population level protection against transmission."
Professor Wendy Barclay, the head of infectious diseases at Imperial, said the findings suggested Covid-19 was likely to work in the same way as the common cold, meaning it could reinfect regularly.
She added: "Seasonal coronaviruses that circulate every winter and cause common colds can reinfect people, after six to 12 months - and we suspect that the way that the body reacts to infection with this new coronavirus is rather similar to that.
"We don't yet know what level of antibody is needed in a person's blood to protect them from infection or reinfection from SARS-CoV-2, but of course that level is a crucial thing to begin to understand."
She said that Britain was "miles off" herd immunity.
Professor Graham Cooke, another Imperial infectious diseases expert, said: "The big picture here is that after the first wave, the great majority of the country still did not have evidence of protective immunity. So although we are seeing a decline in the proportion of people who are testing positive, we still have a great majority of people who are unlikely to have been exposed.
"So the need for a vaccine is still very large if you want to try and get a large level of protection in the population."
