Pubs, bars and restaurants must close at 10pm across England under Boris Johnson's coronavirus rules, but there is an exemption for drinkers in one establishment – Parliament.
The law aimed at slowing Covid-19's resurgence has been criticised by businesses and raised concerns it may do more harm than good by forcing crowds into the streets at the same time.
And it has now emerged that the bars and restaurants frequented by MPs and Lords in the Palace of Westminster are exempt from the regulations.
They will be able to remain open after the curfew because they fall under the description of a workplace canteen, the PA news agency understands.
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 says that "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.
Health Minister Helen Whately said she had been unaware that the curfew did not apply to Parliament and seemed unimpressed.
"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.
Senior Conservative backbencher Steve Baker was also critical, and suggested it would be changed.
"This surely will not last the day, and rightly so," he said.
The curfew that came into force on Thursday has proved controversial, with businesses warning their profitability will be jeopardised and police struggling to disperse large crowds forming 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.
The House of Commons said it would be responding for a request to comment later on Monday.