British Prime Minister Boris Johnson has apologized in parliament for attending his own birthday party at Downing Street two years ago, in breach of coronavirus lockdown rules.

Johnson told members of parliament on Tuesday that it did not occur to him that the gathering could amount to a breach of the rules. He said, "I repeat that was my mistake and I apologize for it unreservedly."

Police are investigating allegations that a series of parties were held in the prime minister's office and other government buildings while strict lockdown rules were in place in 2020 and 2021.

Johnson was fined last week for attending his birthday party in June 2020.

British media reported that it was the first time a sitting prime minister has been fined for breaking the law.

Johnson is brushing aside calls from the opposition to step down, saying he needs to respond to the Russian invasion of Ukraine and other priorities.