Simon Case, who was appointed in September 2020 having served as permanent secretary in No 10 before then, also said the government was looking like a "terrible, tragic joke" and that he "cannot cope with this" in apparent frustration at how the coronavirus pandemic was being handled.

The messages were briefly flashed up on a screen during Friday's hearing of the UK COVID-19 Inquiry in London, as a letter to the inquiry from then prime minister Boris Johnson's adviser Dominic Cummings was shown and discussed.

The hearing is part of module two which focuses on core UK decision-making and political governance.

The letter contained an email dated 13 July 2020 which Mr Cummings said was "on the problems of the No 10/CabOff [Cabinet Office] set up that is relevant to the inquiry".

Mr Cummings said it was copied to Mr Johnson "but he never engaged seriously".

The WhatsApp messages appeared on a screen above the email and are understood to have been from 14 October 2020.

Mr Case wrote: "Am not sure I can cope with today.

"Might just go home. Matt [Hanock] just called, having spoken to PM. According to Matt (so aim off, obvs), PM has asked Matt to work up regional circuit breakers for the North (as per Northern Ireland) today - and to bring recommendations. I am going to scream…"

Lee Cain, who was Mr Johnson's director of communications until November 2020, replied: "Wtf are we talking about."

Responding, in an apparent reference to Mr Johnson's then-partner and now wife, Carrie Johnson, Mr Case wrote: "Whatever Carrie cares about, I guess."

He added: "I was always told that Dom [Dominic Cumming] was the secret PM. How wrong they are. I look forward to telling select cttee tomorrow - 'oh, f*** no, don't worry about Dom, the real person in charge is Carrie'."

In the same series of messages, Mr Cain said: "Also she doesn't know wtf she is talking about either. Jesus."

'I cannot cope with this'

In a subsequent post, Mr Case wrote: "This gov't doesn't have the credibility needed to be imposing stuff within only days of deciding not too (sic).

"We look like a terrible, tragic joke.

"If we were going hard, that decision was needed weeks ago. I cannot cope with this."

Power struggle

The latest disclosures will lead to renewed pressure on Mr Case, who remains in post despite previous embarrassing WhatsApp messages.

Mr Cain and Mr Cummings both left Downing Street towards the end of 2020 following a bitter power struggle with Mr Johnson's then fiancee.

Mrs Johnson, nee Symonds, is a former Tory press chief who has served as a special adviser in previous governments.