The announcement yesterday saw sources close to Mr Sunak add criticism of Brexit Opportunities Minister Jacob Rees-Mogg’s pace in getting rid of decades of regulations inherited from Brussels.
But sources close to Mr Rees-Mogg have insisted that “the biggest blocker of getting rid of EU law was the Treasury headed by the former Chancellor Rishi Sunak.”
The claims have been strongly denied Mr Sunak’s team who insist he was the only minister to deliver Brexit reforms.
Mr Rees-Mogg is one of a number of leading Brexiteers, including the influential European Research Group (ERG), who have backed Ms Truss instead of Mr Sunak even though she voted Remain in 2016 and he backed Leave.
A senior Whitehall source said: “Liz Truss was helpful with every single difficulty we had with Brexit policy, she was always on the right side [in government discussions] and ambitious.
“But Rishi Sunak was always risk averse and on the side of the Treasury mandarins who did not want us to diverge from Europe. MPs know this.”
In particular, Mr Sunak is accused of blocking tax law from being part of the Brexit freedoms legislation.
Allies of Mr Rees-Mogg hope that if Ms Truss wins as expected then some of the tax law and regulations can be brought back into the legislation.
VAT was an EU competence but there were also issues around definitions and regulations.
A Whitehall source noted: “Rishi didn’t want any tax law to change, he wanted us to stick with EU definitions and case law.
“He listened to risk averse Treasury mandarins all the time.
“As Chancellor he was the second most powerful minister and he did nothing to change the orthodoxy in the Treasury.”
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The sources suggested Mr Sunak’s policy of scrapping 2,000 regulations in 100 days was “hard to believe” because of the way he used his old job “to make it an impossibility.”
He was also accused of making it difficult to deliver financial services reform – despite Mr Sunak’s camp claim this was one of his achievements.
The source claimed when the treasury did his Brexit opportunities consultation with departments under Mr Sunak it refused to participate.
When it did it was “like pulling teeth.”
The source added: “Again it wasn’t ambitious enough.
“When he was Chancellor he wouldn’t challenge the Treasury and we can have no confidence he would do it as Prime Minister.”
A spokeswoman for Mr Sunak said: “The only person in Whitehall who delivered on Brexit freedoms in their brief was Rishi. From establishing eight Freeports to ripping up the EU rulebook when it came to financial services, he delivered a proper plan for reform and change.”