Boris Johnson told Dominic Cummings in 2016 that it would be “ludicrous” for him to become Prime Minister, according to the former adviser’s latest blog post.

The Prime Minister’s former chief aide has been using his Substack website to launch a series of attacks on the way the Government operates.

Mr Cummings claimed in a blog post on Monday that he knew that Mr Johnson was “unfit to be PM”, adding: “We also knew that he knew too, since he’d told us.”

Mr Cummings wrote: “On 24 June 2016 in Vote Leave HQ, just after Cameron had resigned, Boris pulled me into the odd little room where the ‘campaign within the campaign’ was run. What now?

“Boris told me with a laugh, ‘Obviously it’s ludicrous me being PM — but no more ludicrous than Dave or George, don’t you think?’

“I agreed and reminded him of the main elements of the deal we’d agreed with (Michael) Gove about what to do next.”

The former adviser left Number 10 in November following a bitter power struggle.

In recent months Cummings has frequently used his blog and Twitter account to criticise the Government he helped into power, reveal damaging information about Johnson and attack former health secretary Matt Hancock.

The former adviser shared WhatsApp messages from Johnson branding Hancock “totally f****** hopeless” in the early days of the pandemic. Weeks later Hancock resigned after leaked CCTV footage showed him kissing departmental aide Gina Coladangelo.

Following Hancock’s resignation last week Cummings claimed Johnson – who initially said he considered the kissing matter closed – only let Hancock go due to being bombarded with text messages from his wife Carrie.

Cummings claimed “89 Carrie texts p/hour” influenced Johnson’s U-turn.

When Hancock’s replacement Sajid Javid was announced, Cummings decided to again stir the pot – saying he “tricked” Johnson into firing the former chancellor into firing him last year and calling the appointment “awful” for the NHS.

He alleged that Javid’s promotion, too, was influenced by Carrie.