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 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.
This comes after it was reported last month that Johnson – furious at having to impose a second lockdown – said he was prepared to let “bodies pile high” rather than order a third one.
The prime minister claimed that allegation was "total rubbish".
As coronavirus infections started to increase in October last year, Johnson faced repeated calls to impose a second lockdown, but chose to persevere with the controversial tier system of localised restrictions.
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.
He is set to give evidence at 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.