Matt Hancock's former aide called Dominic Cummings a "f****** piece of s***" as he accused the ex-adviser of "relentlessly" briefing against the former health secretary, the latest set of leaked WhatsApp messages reveal.
Jamie Njoku-Goodwin made the remark on the same day Mr Cummings, who had been Mr Johnson's most senior adviser, used a parliamentary committee hearing to describe the Department of Health and Social Care as reduced to a "smoking ruin" by the pandemic.
Mr Hancock, who was health secretary during much of the pandemic, messaged on the day of the hearing in March 2021: "How would you deal with this Cummings crap?"
His former adviser responded privately: "I was about to message. What a f***ing piece of s***. You went out and backed him over Barnard castle, and he responds by briefing against you relentlessly, in private and now in public. He's a psychotherapist."
He swiftly corrects himself: "Psychopath."
The government was forced to defend Mr Cummings after he drove to County Durham beauty spot Barnard Castle during the first lockdown.
In May 2020, the WhatsApp messages show Mr Hancock had requested that Mr Njoku-Goodwin "draft a supportive tweet" after calls for Mr Cummings' resignation over the lockdown breach.
The latest set of leaked messages also reveal then-chancellor Rishi Sunak told Mr Hancock that Mr Cummings' time in Downing Street was a "nightmare".
The remark was made on the day former Number 10 aide Mr Cummings used an appearance in front of MPs to claim thousands of people needlessly died during the pandemic.
Mr Hancock messaged Mr Sunak: "Of all the bonkersness (sic) about Dom's circus, the one I enjoy most is that he's doing this to secure his place at the heart of the future Sunak administration."
Mr Sunak, who in 2021 was seen as ambitious for the top job, said: "Ha! Ironic given I haven't spoken to him since he left!"
"It's just awful & a stark reminder of how hard governing was," Mr Hancock replies.
Mr Sunak agrees: "It was such a difficult time for all of us. A nightmare I hope we never ever have to repeat."
The details emerged in the latest tranche of leaked messages from former health secretary Mr Hancock, published by the Daily Telegraph.
Other messages show that on a number of occasions, Mr Hancock expressed concern that Mr Sunak's signature Eat Out to Help Out initiative was contributing to the spread of COVID-19 - dubbing it the "eat out to help the virus get about".
Concerns have been expressed subsequently that the scheme could have contributed to the spread of the virus.
The messages also show Mr Hancock attempting to get the support of Cabinet Secretary Simon Case in challenging the stance of Mr Sunak and others over certain pandemic-era rules, with the top civil servant - who is required to be politically neutral - complaining about "pure Conservative ideology" on the part of one senior minister.
In one message from October 2020, Mr Hancock appears to hit out at Mr Sunak's attitude about lockdowns, writing: "What's Rishi's dilemma? Whether to stop the virus, or tilt at the party & show ankle to the hard right?"
The exchanges were among more than 100,000 messages passed to the Telegraph by the journalist Isabel Oakeshott.
She was originally given the material by Mr Hancock while they were collaborating on his memoir of his time in government during the pandemic.
Mr Hancock has condemned the leak as a "massive betrayal" designed to support an "anti-lockdown agenda".
In a statement this week, Mr Hancock said that all the materials for his book have been made available to the official COVID-19 inquiry.
Ms Oakeshott has said the disclosures are in the public interest.
The paper also published messages showing Mr Hancock and his officials scrambling to save the health secretary's career after footage emerged of his embrace with aide Gina Coladangelo.
A spokesman for Mr Hancock said: "There's nothing new in these messages, and absolutely no public interest in publishing them given the independent inquiry has them all. It's highly intrusive, completely inappropriate and has all been discussed endlessly before."