Downing Street has rejected as “a highly defamatory fabrication” a claim that Boris Johnson’s chief adviser, Dominic Cummings, initially argued against strict measures to contain coronavirus in a view summarised as “if that means some pensioners die, too bad”.

In an unusual on-the-record denunciation, a Downing Street spokesman said the claims about Cummings’ viewpoint, made in a Sunday Times article, had not been put to No 10 in advance and contained “invented” quotes.

The report claimed that at one private event at the end of February, Cummings outlined then government’s strategy at the time in a way that was summarised by some present as “herd immunity, protect the economy, and if that means some pensioners die, too bad.”

The allegations, which have been widely shared online, connect with wider criticisms that the government response to the virus was initially too weak, based on a notion that rather than limiting its spread, enough people could be allowed to contract it to give population-wide “herd immunity”.

While the phrase “herd immunity” was used by government figures such as Sir Patrick Vallance, the chief scientific adviser, No 10 has denied it was ever a tactic. However, the use of the phrase illustrated initial tensions within government over how to balance the economic impact of a full national lockdown against the potential number of deaths from the virus.

According to the Sunday Times report, the key moment came on 12 March when a group of government experts gathered to examine modelling of the spread of the virus carried out by academics at Imperial College London and elsewhere.

This predicted that if no action was taken more than half a million people would die, and that even some limited mitigation efforts would only halve this. The Sunday Times report said this changed the mind of Cummings, who before had been an adherent of the herd immunity idea.

After the 12 March meeting, Cummings changed his view and became one of the strongest advocates in government for tough restrictions to curb the spread of the virus, the Sunday Times said. It quoted one anonymous senior Conservative as saying: “He’s gone from ‘herd immunity and let the old people die’ to ‘let’s shut down the country and the economy’.”

The Downing Street spokesman said: “This is a highly defamatory fabrication which was not put to No 10 by the Sunday Times before publication. The article also includes a series of apparent quotes from meetings which are invented.”

Cummings is already a controversial figure within government. The former Vote Leave strategist is regularly portrayed as an ideologue and hidden power over Johnson.