Two fast-food takeaways have been fined £1,000 for taking orders and serving food after the 22:00 curfew.
Oxford City Council fined a KFC branch and Bodrum Kebab in Cowley Road after staff were "caught" serving customers at their premises.
Council officers said they witnessed KFC taking orders after hours on three dates and Bodrum on one occasion.
The businesses are the first to be fined by the local authority's so-called "Covid secure team".
The council said coronavirus laws state that although takeaways can continue operating after 22:00 using a delivery service, click-and-collect or drive-through, the law forbids them from taking orders and serving food in their premises or at their door after that time.
Officers investigated after complaints from the public, the council said.
A manager at Bodrum Kebab denied any wrongdoing after being contacted by the BBC.
She said: "It's not true. We served customers who have a car outside. [We] are allowed to sell to customers with vehicles outside. Some people are jealous that we are getting customers."
The manager, who did not to be named, said she could not confirm whether the restaurant had paid the £1,000 fine or was intending to.
KFC declined to comment when contacted by the BBC.
'Making the city less safe'
The city council has the power to fine businesses up to £10,000 for breaking the coronavirus laws, but chose to issue a smaller fine "as this was a first offence for both businesses".
Councillor Louise Upton said many bars and pubs in the city centre had "gone above and beyond to ensure the safety of their staff and customers".
"But any businesses that break the coronavirus rules are irresponsibly making the city less safe for everyone, and they should know that we will take action against them," she added.
All six Oxfordshire councils have set up Covid secure teams to ensure businesses comply with coronavirus laws.