Dominic Cummings has vowed to lift the lid on why ministers delayed forcing people to wear face masks to fight Covid-19 last year, as MPs accused him of profiteering from the chaos in 10 Downing St.

Health Secretary Matt Hancock was challenged by MPs last week over the five-week delay over the decision after scientists had recommended face coverings last May.

In a short reference in a 7,000 word blog this week, Mr Cummings claimed that Mr Hancock had given a "fictitious account" of what had happened on masks and pledged to "leave it to another day".

The blog – which included personal WhatsApp messages from Boris Johnson, the Prime Minister – was published on the substack website where visitors are asked to pay up to £200 a year to read Mr Cummings’s other views on business and commerce.

This has prompted Tory MPs to claim that he was using the blog to drive people to the site where he can earn money.

Another said that publishing messages from Boris Johnson risked prejudicing the public inquiry when it starts next year.

MPs on the Health and Business committees asked Mr Hancock last week why – when scientists on the Sage committee had recommended face masks in enclosed spaces on May 11 – the UK Government had only "mandated their use on 15 June in public transport settings, 24 July in shops, and 24 September elsewhere".

Mr Hancock replied that there was "a rigorous international debate about the use of face masks, and there were very significant divisions in the science on this".

This came down to "disagreements about the most likely route of personal transmission of the virus from one person to another".

"There is now a common understanding much more about aerosol transmission from one person, and how face masks are therefore more important than was the initial advice," he said. "There is also a practical consideration, that we did not want people to take face masks away from where they were absolutely vital, within health and social care settings.

"When we did introduce rules around the public’s use of face masks, we introduced them saying that you should wear a face covering, as many people in this room are, because in that way it did not take away from the provision of PPE.

"Now, that is not a problem; we have huge stockpiles of it, but it was a problem at the time."

In his blog on Wednesday – which was titled "some evidence re my and Hancock’s evidence to MPs" – Mr Cummings appeared to call this into question, saying: "Hancock has also given a fictitious account of what happened on masks but I’ll leave that to another day."

Separately, Conservative MPs yesterday demanded to know why Mr Cummings appeared to be able to use his access to decision making in 10 Downing Street to make money.

Mr Cummings’s blogpost was published on the substack website where visitors can pay up to £200 a year to read his other non-government comments.

In a post, Mr Cummings said while "a lot of stuff on covid will be free, some stuff on my time in No10 will be free, some subscriber-only".

He said he wanted to "publish evidence" that would help force ministers to order a public inquiry "as soon as possible". 

Sir Iain Duncan Smith, the former Tory leader, said it appeared Mr Cummings was using his account of his time in Government "to drive [people] to a site where he can earn money".

He said: "It is quite wrong that anyone should be able to profit from what was essentially privileged information that he had the greatest and highest access to."

Sir Iain also asked whether the disclosures of the WhatsApp messages could be breaches of the Official Secrets Act.

Conservative MP William Wragg, who chairs the Public Administration and Constitutional Affairs select committee, said he was worried the disclosures could damage the work of the promised judge-led inquiry into the pandemic.

He said: "My main concern is prejudicing a future inquiry, if your motivation seemingly is just to discredit."

Mr Cummings declined to respond to requests for comment from The Telegraph.

Mr Hancock’s press team was approached for comment.