The former New Labour cabinet minister tweeted an uncorrected proof of Michel Barnier’s upcoming memoirs of the negotiations entitled “My Secret Brexit Diary”. Alongside an image of the novel, he wrote: “Today’s book.

       “Reinforces my view that Brexit was a catastrophe for Britain - (Mr) Barnier was better than all Britain’s negotiators put together and ran rings around Lord Frost.”

       His highly controversial statement quickly attracted scores of comments - including from Boris Johnson’s former chief advisor.

       The maverick political strategist rubbished his claim and said top Eurocrat Stephanie Riso was really the brains behind the EU’s team.

       He said: “Usual ignorance from (Lord) Adonis.

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       “Barnier was an emotional/bullsh***er/lazy/DDavis type.

       “Our real opponent, smart, subtle, tough, perceptive re(garding) PM (Mr Johnson’s) idiocy was Step(hanie) Riso.

       “Lucky for us (esp at end when PM was caving on many fronts) Barnier often sidelined her.”

       Mr Cummings was not the only user to slam the Labour peer’s post.

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       Robert G Mackenzie wrote: “I voted Remain but now pleased we left.

       “Regrettable that Labour, SNP and Lib Dems conspired to side with the EU during negotiations.

       “(In the) meantime good to see IMF's latest World Economic Outlook showing UK to easily remain Europe's 2nd largest economy and outperforming (the) Eurozone.”

       Steve Breave added: “In your opinion? Perhaps if he'd concentrated on negotiations and not on a book deal....still, he's a politician.

       “Why wouldn't he be thinking about his personal wealth in the middle of such important negotiations?”

       In a bio of Ms Riso - who now works for EU Commission President Ursula von der Leyen - Politico wrote she “won a reputation for extreme competence from both sides of the Brexit debate”.

       It added: “Before Brexit, (Ms) Riso was in charge of preparing the EU’s next long-term budget.

       “She has never issued a comment or statement in her own words on Twitter but religiously retweets her boss and colleagues.

       “(Ms) Riso has a reputation as a serious hardworking economist, who is especially appreciated by the British Brexit negotiating team, some of who told POLITICO they prefer her to (Mr) Barnier.”

       Since stepping down from the EU, Mr Barnier has thrown his hat into the ring for next year’s French presidential election.

       Last month he began running for The Republicans presidential nomination.