Former Conservative minister Jacob Rees-Mogg has accused Rishi Sunak of breaking his word after the government ditched plans to allow thousands of EU-era laws to expire by the end of 2023.
Defending the move, Business Secretary Kemi Badenoch told MPs the government was still "ending EU supremacy" but just "changing how we are doing it".
She said it showed Brexiteers could be "pragmatic" and "do what is right".
But Mr Rees-Mogg said the deadline would "make Whitehall work".
Speaking to the BBC's Today programme, he said: "It is hard enough to motivate Whitehall at the best of times - they are not necessarily coming into the office, they don't seem to be working with the efficiency one would like.
"Without a deadline, nothing will happen and we will retain these EU laws for a long time."
Mr Rees-Mogg,who championed the deadline when he was business secretary last year, said getting rid of the laws would help make the UK's economy more competitive and reduce inflation. Of the PM, he said: "He has broken his word. This is very serious in my view".
Dave Penman, the head of the FDA Union which represents senior civil servants, hit back at suggestions the civil service were to blame, saying the deadline was "an inevitability".
"It was a bizarre way of doing business in government to say that unless we get to a certain point in time, any piece of legislation will simply fall away."
During his unsuccessful bid to be Conservative leader in the summer, Mr Sunak sought to attract members' votes by putting out a campaign video which saw bundles of EU laws being shredded.
Mr Rees-Mogg is not the only Conservative MP upset at the government's decision to scrap the 2023 deadline.
On Wednesday, 20 Tory backbenchers went to see the chief whip Simon Hart to express their concern, and some MPs went into Downing Street to do the same.
And during an urgent question in Parliament on the subject, several Conservative MPs criticised the move.
"What on earth are you playing at," asked Mark Francois as he accused the government of performing a "massive climbdown".
Dominic Raab - who resigned as justice secretary last month - urged ministers to "resist the resistance" in Whitehall.
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However, ministers received support from other Conservative backbenchers - Sir Bob Neill said he now felt more able to support the bill, because gaps in the legislation had been "sensibly filled".
The divisions in the Conservative Party over Brexit are far from as serious as they were under Theresa May. But there are still spits over the pace and extent of divergence from Europe.
When the UK officially left the EU in 2020, the UK incorporated thousands of EU laws into UK law to minimise disruption to businesses - with an ongoing audit by civil servants having identified 4,800 so far.
The Retained EU Law Bill, introduced during Liz Truss's premiership, set a 31 December 2023 deadline, after which most of the laws would have expired unless ministers decided to replaced or retain them.
Critics - including opposition parties, trade unions and campaign groups - had argued that the deadline was unrealistic and could lead to important legislation being lost by accident.
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Environmental groups had been particularly concerned warning about a loss of rights and legal protections in areas including water quality, air pollution standards and protections for wildlife.
Setting out the decision on Wednesday to scrap the deadline, Business Secretary Kemi Badenoch said the cut-off point would be replaced with a list of 600 laws the government wants to replace by the end of the year.
Speaking to MPs, Ms Badenoch said the government had revoked over 1,000 pieces of rules and would repeal a further 2,000 by the end of the year.
Labour's shadow Cabinet Office minister Justin Madders, described the situation as an "absolute shambles".
"It was completely unrealistic, reckless and frankly arrogant to think they could strike 4,000 laws from the statute book in the timescale."
The SNP's Kirsten Oswald's described the bill as "damaging" and "anti-democratic" and expressed concern that UK ministers would still have the power to act in areas that are devolved to Scotland.