The singer says she attended a party at a restaurant in west London on Saturday.
"Given the restrictions, I realise how irresponsible these actions were and I take full responsibility," she wrote in a statement.
It's reported up to 30 people were at the party, although she describes the event as a "small gathering".
She says it was a "spur of the moment" decision.
London's Metropolitan Police confirmed officers went to the Casa Cruz restaurant after a report about a potential breach of Covid lockdown regulations.
"I'm deeply sorry for breaking the rules and in turn understand that this puts people at risk," she posted.
"This was a serious and inexcusable error of judgement."
Current lockdown regulations in England mean you can only meet one other person outside.
Pubs and restaurants are currently closed (although they can serve non-alcoholic takeaways) and you shouldn't be with people from outside your house or support bubbles indoors.
Police have the power to break up groups larger than six and those who ignore officers could be fined £100, doubling with each offence to a maximum of £3,200.
So far, the singer has not been fined.
Kensington and Chelsea Council, which is responsible for issuing Covid fines against businesses in the area, says it is still investigating what happened at Casa Cruz.
The singer's not the first high-profile public figure to admit breaching lockdown. In October, Chelsea striker Tammy Abraham apologised "for the naivety shown" in attending a party for his 23rd birthday.
In May, Manchester City's Kyle Walker wrote to the club's supporters after breaching coronavirus restrictions.