The theory was quickly dismissed at the time by scientists as a crank idea with no foundation.
Mr Johnson asked the government’s chief scientific adviser Sir Patrick Vallance and chief medical officer Prof Chris Whitty what they thought of the idea.
The theory, which was also circulated on social media, was considered almost as ludicrous as when former US president Donald Trump suggested injecting bleach into the body to fight the coronavirus.
Such ideas were regularly featured in lists of Covid myths published by the media during the pandemic.
Mr Cummings is one of several former advisers and officials to Mr Johnson who have lined up to slam his leadership during the Covid inquiry.
In a 114-page statement of evidence, the former strategist – who was sacked in November 2020 after losing a bitter power struggle in No 10 – lays out a torrent of criticism of the government he served.
He says Mr Johnson’s circulating the hairdryer video was “a low point” amid fears he could have been giving false information to the media. The former top adviser, wrote: “We also couldn’t be sure that he was not himself the source of false stories.”
He cites a case when it was reported that the government “knew masks didn’t work”. “This was the opposite of the truth,” Mr Cummings wrote. “Some thought the PM was the source of the story.”
Mr Johnson also told Mr Cummings in the autumn of 2020 that he wanted him to “dead cat” Covid – find another big story to distract the public – because he was “sick” of the issue. The adviser told the PM that this would not work.
Mr Cummings also said Mr Johnson had to be stopped from going to see the Queen on 18 March - five days before the first lockdown. “I was desperate and said something like, ‘If you’ve got Covid and you kill the Queen you’re finished’.
Mr Cummings claims that Carrie Johnson exacerbated the then-PM’s indecisiveness. But he also claims that Mr Johnson himself sometimes blamed her unfairly for U-turns that were “NOT her fault”.
He also repeated a suggestion that Mr Johnson was working on a book about William Shakespeare rather than the pandemic during a two-week holiday in February 2020.
Elsewhere, he writes that “the PM was in a terrible mental state (eg ranting ‘I’m the f****** Fuhrer around here’) over illegal donations”.
Mr Cummings, who is highly critical of everyone from the media and officials to ministers, the Cabinet Office, says in a discussion on national security that the cabinet was “largely irrelevant to policy or execution in 2020”.
On Covid planning, he writes: “The pandemic plan was bad. It looked narrowly at flu … Any proper process would immediately have raised things like the absence of a shielding plan.”
The former PM has insisted his actions during the pandemic, including lockdowns, saved thousands of lives.