The research, among 2,847 key workers from the NHS, police and fire service in June, found that 25 per cent of participants had high levels of T-cells which recognised Covid.
That is far higher than the results of antibody surveys, which have consistently found no more than six per cent test positive for having had Covid.
And it is likely to be even higher now that the country has experienced a second wave of infections.
Experts at Cambridge University’s MRC Biostatistics Unit estimate 7.37million people have caught the coronavirus already in England, and therefore would likely have immunity.
The PHE study, however, suggests the proportion of people with some level of immunity could be around one in four, which would equate to twice as many people at 14million or more.