Boris Johnson’s former chief aide has described the prime minister as a “gaffe machine” who is “clueless about policy and government”.

Dominic Cummings continued his assault on the government and the prime minister as he also attacked “drone-babblers” among Westminster pundits.

The ex-adviser, who has been engaged in a weeks-long feud with Downing Street, made another intervention following the Conservatives’ by-election loss to the Liberal Democrats in Chesham and Amersham.

Mr Cummings urged people to “stop reading these pundit babblers” as he listed examples of media predictions – such as an assured Conservative victory in Chesham and Amersham – that turned out to be wrong.

He also highlighted the past controversy of Mr Johnson not doing an interview with journalist Andrew Neil during the 2019 general election campaign, for which the prime minister was heavily criticised.

Mr Cummings, who worked for the prime minister during the campaign, pointed out how it was suggested that Mr Johnson not doing the interview was a “huge campaign blunder”.

Posting on Twitter, he added: “Why the fu*k wd be put a gaffe machine clueless about policy & government up to be grilled for ages, upside=0 for what?! This is not a hard decision…

“Pundits don’t understand comms, power or management. Tune out!”

Downing Street later said it did not accept the description of Mr Johnson as a “gaffe machine”.

“Of course that is not a characterisation that we would accept,” a spokesman for the prime minister said.

“But I’m not going to get into specific allegations.”

Mr Cummings has made a series of explosive claims about Mr Johnson since leaving Downing Street last November amid a Number 10 power struggle.

Earlier this week, he published a 7,000-word blogpost in which he included screenshots of WhatsApp messages in which Mr Johnson referred to Health Secretary Matt Hancock as “totally f****** hopeless”.

But the prime minister has since given his full backing to Mr Hancock.

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“I have complete confidence in Matt and indeed all of the government who have been dealing with COVID-19 during the pandemic,” Mr Johnson said on a visit to Kirklees, West Yorkshire, on Friday.

Mr Cummings has promised to give answers to an “ask me anything” opportunity on his blog next Monday, although it will only be available for paying subscribers.