Keir Starmer has demanded Boris Johnson publish the scientific evidence behind the 10pm curfew ahead of a crunch Commons vote next week on the law.
Speaking during PMQs on Wednesday, the Labour leader gave a strong indication he may withhold his support.
If Labour joins Tory rebels, there is a chance the government could be defeated in any bid to keep the curfew in place.
“One question is now screaming out: is there a scientific basis for the 10pm rule?” Starmer said.
“If there is, why doesn’t the government do itself a favour and publish it? If not, why doesn’t the government review the rule?”
He added: “Will the prime minister commit to publishing the scientific basis for the rule before this House votes on it?”
Johnson sidestepped the call for any science behind the curfew to be published and told MPs the point of the law was “to reduce the spread of the virus”.
The government has been under increasing pressure to scrap the curfew from Tory MPs and the hospitality industry.
Steve Baker, the leading backbench rebel, told HuffPost UK: “It is not clear what the evidence is to support the 10pm curfew or that it is effective.”
Kate Nicholls, the CEO of UK Hospitality, said the curfew and other restrictions had a “severe and devastating” impact on pubs, restaurants and other venues.
Pub giant Greene King said on Wednesday it plans to cut around 800 jobs and shut dozens of pubs and restaurants
Local Labour leaders have also warned the curfew is counter-productive as it has led to people all leaving bars at the same time, gathering together outside, holding more house parties and cramming on to public transport.
HuffPost UK revealed today ministers are considering shifting the curfew back an hour, with supermarkets being ordered to stop selling alcohol after 11pm.
The plan would transplant the rules operating in Northern Ireland to England, which see last orders being called at 10.30pm.
During PMQs, Starmer also said Labour analysis showed 19 out of 20 areas in England that have been under restrictions over the last two months have seen an increase in infection rates anyway.
Bolton, which has been under restrictions since July 30, has seen its infection rate increase almost 13 times from 20 to 255 per 100,000.
Burnley, which has been under restrictions since July 31, has seen its infection rate increase over 20 times from 21 to 434 per 100,000.
Bury, which has also been under restrictions since July 31, has seen its infection rate increase over 13 times from 20 to 266 per 100,000.
An analysis of government data by HuffPost UK also showed there are now only seven areas of the UK with levels of Covid-19 under the government’s own threshold for foreign countries that require travel restrictions.
People visiting countries with more than 20 cases per 100,000 people over a seven-day average are required to quarantine for 14 days upon their return to the UK.