Bars and restaurants in the Houses of Parliament will stop serving alcohol at 10pm, despite a loophole around workplace canteens meaning they continue serving past the curfew which affects all hospitality venues in England.
The law has received sharp criticism from businesses, with many suggesting it may do more harm than good by forcing crowds into the streets at the same time.
The news that Parliament's establishments were exempt added to frustrations.
They were able to remain open after the curfew because they initially fell under the description of a workplace canteen, the PA news agency understands.
After the news became public, a UK Parliament spokesperson issued a statement outlining a u-turn on the policy:
"Alcohol will not be sold after 10pm anywhere on the parliamentary estate," it said.
Catering facilities that do not sell alcohol will be allowed remain open later when the House of Commons is sitting, to serve food for those still working.
The regulations, announced by the Prime Minister last week, include exemptions for cafes at hospitals, care homes and schools, as well as those providing food to the homeless.
It also states "workplace canteens may remain open where there is no practical alternative for staff at that workplace to obtain food".
In the centre of London there are relatively few shops surrounding Parliament, particularly late at night, and politicians and staff tend to eat on site.
Speaking ahead of the U-turn, Health Minister Helen Whately said she had been unaware that the curfew did not initially apply to Parliament and seemed unimpressed at the news.
“We in Parliament shouldn’t be sitting round late at night drinking. We have got a job to do when we are there,” she told BBC Radio 4’s Today programme.
The curfew that came into force on Thursday has proved controversial so far.
Businesses have warned profitability will be jeopardised and police have struggled to disperse large crowds forming in city centres after the deadline on Saturday night.
Greater Manchester mayor Andy Burnham has also warned it may be doing “more harm than good”, with people piling on to public transport and queuing outside shops to buy more alcohol.