Rishi Sunak’s vow to scrap hundreds of remaining EU laws and regulations has been undermined by leaked Treasury documents from his time as Chancellor, warning of the dangers of doing just that.

On Sunday, in a bid to woo Brexiteers, the frontrunner in the contest said, if elected, he would appoint a Brexit minister to go through the remaining 2,400 EU laws still on the statute book.

They would then have to come up with a list of rules to be scrapped within Mr Sunak’s first 100 days in office.

In a statement setting out his proposals, Mr Sunak stressed that he had always backed Brexit, even though he had been warned it could damage his political career because it was the “right thing” for the country.

Mr Sunak said: “We need to capitalise on these opportunities by ditching the mass of unnecessary regulations and low-growth mentality we’ve inherited from the EU.

“I have a plan, if elected prime minister, to have scrapped or reformed, by the time of the next election, all the EU law, red tape, and bureaucracy still on our statute book that is holding back our economy.

“As prime minister, I would go further and faster in using the freedoms Brexit has given us to cut the mass of EU regulations and bureaucracy holding back our growth.

“If we do this, we can get our economy growing quickly again and become the most prosperous country in Europe.”

The review would include an overhaul of the remaining EU financial services regulations with a view to triggering a “Big Bang 2.0” for the City to enable it to maintain its status as a leading world financial centre.

Mr Sunak also indicated that he would overhaul EU General Data Protection Regulation (GDPR) rules and speed up the clinical trials by cutting EU red tape.

However, papers, obtained by Bloomberg, show that a senior Treasury official working for Mr Sunak when he was still Chancellor said EU-derived tax laws should be exempted from any legislation to scrap so-called retained EU law.

Lucy Frazer, the financial secretary to the Treasury, said in a letter that it was not feasible to simply rip up EU laws that had been integral to British tax policy and case law for four decades. 

She argued that the Treasury should take charge of more carefully repealing the laws during future finance bills to prevent the government from becoming mired in litigation. She also suggested it would not be possible to sunset retained EU law by 2026.