Dominic Cummings will claim Boris Johnson said "COVID is only killing 80-year-olds" ahead of the disastrous second wave, according to a report.
ITV has reported that Cummings, at his long-awaited select committee appearance on Wednesday, will say the prime minister made the remark as he argued against the second national lockdown, which was ultimately imposed on 5 November.
As coronavirus infections started to increase in October last year, Johnson faced repeated calls to impose another national lockdown, but initially chose to persevere with the controversial tier system of localised restrictions.
It was also reported last month that Johnson – furious at eventually having to introduce the second lockdown – said he was prepared to let “bodies pile high” rather than order a third one.
Watch: PM tells Commons he did not make "bodies pile high" comment (from 28 April)
The PM claimed that allegation was "total rubbish". He later imposed a third lockdown on 4 January.
Government data show daily COVID-19 deaths in the second wave peaked at 1,358 on 19 January. As of Tuesday, there had been 127,739 deaths in total.
Cummings was Johnson's chief adviser between July 2019 and November last year, when the pair fell out amid a reported power struggle in Downing Street and Cummings left.
He is set to give evidence at a joint meeting of the House of Commons health and social care and science and technology committees from 9.30am on Wednesday.
Cummings, whose public statements against Johnson and the government have become increasingly hostile in the past two months, has previously hinted there will be explosive revelations at his committee hearing.
At one point last week, Cummings claimed he has the "only copy of a crucial historical document from COVID decision-making".
A Sunday Times report also suggested officials fear Cummings will use the committee appearance to accuse Johnson of missing key meetings on the COVID crisis because he was working on a biography of Shakespeare, because he needed the money to fund his divorce from Marina Wheeler, his second wife.
Johnson's official spokesman said on Monday he was “not aware” of the PM doing any work on a biography of Shakespeare while in office.
Watch: 'Good try': Dominic Cummings refuses to comment on Boris Johnson (from 19 May)