Boris Johnson is still secretly seeking to create ‘herd immunity’ to defeat the Covid-19 pandemic, a former chief scientific adviser has suggested – despite official denials.

David King said hints that the lockdown will be eased provided the NHS is not “overwhelmed” pointed to the government still expecting most people to catch coronavirus and develop antibodies.

The herd immunity theory was floated by advisers in early March, but appeared to be dropped when ministers were warned the UK death toll could reach a terrifying 250,000.

But Sir David, the scientific adviser between 2000 and 2008, said ministers appeared to have “softened” their tests for starting to relax restrictions, saying: “Maybe we are going for herd immunity?

“In other words, maybe the policy is to allow the virus to spread so that we have a large proportion of our population who have antibodies and, at that point, we will all be resistant to the virus and the lockdown can be removed?”

Sir David also fiercely criticised the government’s failure to crack down hard on the virus at the start, which had reaped rewards from South Korea to Germany to Greece.

“It’s extra sad because we are the country that has the leading scientists and medical experts etc, who could have advised us very early on to raise our capacity to manage what would become a massive pandemic in this country with a large death rate,” he told BBC News

“At one point we heard that 20,000 [deaths] would be a good number – it would be interesting to know what is considered to be a good number now?”

Patrick Vallance, Sir David’s successor, put the phrase herd immunity in the public domain in mid-March, when he warned coronavirus would become “an annual seasonal infection”.

“Communities will become immune to it and that’s going to be an important part of controlling this longer term. About 60 per cent is the sort of figure you need to get herd immunity.”

The controversy has dogged ministers ever since. Only on Wednesday, Michael Gove told MPs: “It was never government policy.”

“It is a term that epidemiologists and others use to describe a particular stage in the development of a disease, if a particular set of principles are followed. But it’s not government policy.”

Sir David said a key test of what is the government’s strategy would be the level of the ‘r’ reproduction rate – how many people are being infected by each virus victim – when the lockdown easing begins.

Anything below 1 means the virus is in decline. He suggested it was currently “between 0.6 and 0.95” – but would need to be “well below 0.6” to prevent a second surge.

No 10 has strongly denied any softening of its tests, after the most important one was described as phrased as ensuring no risk “of a second peak of infections that overwhelms the NHS”.

The last four words were missing earlier in the week – but were uttered by Dominic Raab when the tests were first set, two weeks ago.