Dominic Cummings has called Piers Morgan "a vain tool shouting angry soundbites" in a Twitter spat over MPs boycotting Good Morning Britain.
The prime minister’s former chief adviser hit back at Morgan after the ex-GMB host criticised him for not allowing Tory MPs to appear on the show.
The ban was eventually lifted when former health secretary Matt Hancock appeared on the ITV 1 programme last November after Johnson’s communications chief Lee Cain had left.
Cummings also quit his position in November.
On Tuesday, Morgan angered Cummings after reacting to Sky News footage of him calling the PM a ‘joke’ over his handling of the pandemic.
He wrote: “REMINDER: it was Cummings (and his Muttley, Lee Cain) who ordered the 8-month ban on ministers appearing on @GMB because we were holding them to ferocious account for all the deadly mistakes they were making last year in the pandemic.”
Cummings replied: “I banned them cos we all had better things to do, like control the fkd [shopping cart], than waste our time on a vain tool shouting angry soundbites and calling it ‘news’.”
Morgan then suggested Cummings was vain himself, adding: “a) I don’t think you’re in any position to throw around the ‘vain tool’ slur…
“b) Good to see you finally admit you stopped me holding ministers to proper account for their failures that cost 1000s of lives. Gutless & pathetic, like your lies over Barnard Castle.”
In May last year during lockdown, Cummings defended his decision to drive 250 miles from London to Durham to access childcare after fearing he had contracted COVID-19 and a further excursion to nearby Barnard Castle to apparently test his eyesight as part of his recovery.
The spat came in Cummings' latest Twitter outburst, in which he claimed Johnson never understood what his Withdrawal Agreement with the EU really meant.
He said he had always intended to get “the trolley” – his derogatory nickname for the prime minister – to “ditch the bits we didn’t like” after beating Labour in the 2019 general election.
His latest intervention came after Brexit minister Lord Frost set out the UK’s demands for fundamental changes to the Northern Ireland Protocol in the agreement Johnson signed in January 2020.
He said when Johnson finally realised the true implications of the deal, he said he would never have agreed to it – although Cummings said that was a lie.
During the election campaign, the PM repeatedly boasted the “divorce” settlement he had negotiated with Brussels – including the Northern Ireland Protocol – was a “great” deal that was “oven-ready” to be signed.
Cummings said: “What I’ve said does NOT mean ‘the PM was lying in General Election 2019’, he never had a scoobydoo what the deal he signed meant.
“He never understood what leaving Customs Union meant until November 2020.”
Cummings, who was credited with masterminding the successful Vote Leave campaign in the 2016 referendum, added when Johnson entered No 10 in 2019, the country was facing the “worst constitutional crisis in a century” with much of what he called the “deep state” angling for “Bino” (Brexit in name only) or a second referendum.
He added: “So we wriggled through with best option we could and intended to get the trolley to ditch bits we didn’t like after whacking (Labour leader Jeremy) Corbyn. We prioritised.”
Watch: How the world could be better after COVID