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The former director of the Vote Leave campaign has condemned the Government’s handling of Brexit and said that triggering Article 50 was a “historic, unforgivable blunder”.

Dominic Cummings, one of the masterminds behind the infamous slogan claiming the UK sends the EU £350m a week, said the Government’s approach to Brexit had been a “shambles”.

Mr Cummings, an ally to Foreign Secretary Boris Johnson and Michael Gove during the EU referendum, accused the Government of triggering Article 50 without a plan and said it was like “putting a gun in mouth and kaboom”.

He claimed the Brexit Secretary David Davis and his team had “listened to bullsh** legal advice and led the British people like “lambs to the slaughter”.

In a series of tweets posted on Monday, he said: “Vote Leave said: do not commit to using A50 (Article 50). DD (David Davis) et al listened to bullsh** legal advice and led like lambs to slaughter by (Cabinet Secretary Jeremy) Heywood.

“I said triggering A50 quickly without plan and legal preps for no deal would be like putting a gun in mouth and kaboom. DD chose to do exactly that.

“The shambles now unfolding is a direct consequence of that historic unforgivable blunder.”

His comments came after the Evening Standard revealed that the top official at the Department for Exiting the European Union was stepping down after a year in the job.

The decision of Oliver Robbins to move from Permanent Secretary at DExEU to a co-ordinating role at Downing Street in the middle of Brexit negotiations has plunged Theresa May’s Brexit plans into further disarray.

Mr Cummings added that at the next Cabinet meeting, all secretaries of state should demand a full briefing from Mr Davis and his team on their contingency plans if a Brexit deal is not reached.

Mr Cummings, a former adviser to Mr Gove when he was Education Secretary, then said: “The sooner MPs realise where DD has driven them the sooner mitigation can begin.”

In a previous series of tweets, the former Vote Leave campaign director called Mr Davis as “thick as mince” and “lazy as a toad”.

He also said leaving Europe could be an “error”, described the EU referendum as a “dumb idea” and said other options should have been tried out first before the vote was called.

As the leading strategist for Vote Leave, Mr Cummings coined the group’s official slogan “vote leave, take control”. An official Vote Leave poster also claimed that “Turkey (population 76million) is joining the EU.”

He also came up with the campaign guaranteeing an extra £350m would be spent on the NHS every week from diverted EU spending. The claim was emblazoned across the side of the Vote Leave campaign bus.

Over the weekend, Mr Johnson had a 4,000 word article published in The Daily Telegraph which set down his personal demands for a hard Brexit and repeated the claim that Britain could gain control over £350 million a week gross contributions to Brussels.

The chairman of the UK Statistics Authority Sir David Norgrove accused Mr Johnson of a “clear misuse” of official figures.