Boris Johnson’s father said he was “extremely sorry” after being pictured in a London newsagents on Tuesday without a face covering.
Face masks have been compulsory in shops and supermarkets in England since July.
The picture of the UK prime minister’s father emerged after fines increased to £200 last week for first time offenders who fail to wear a face covering in accordance with government guidance.
Speaking to the Mirror, Mr Johnson Sr said he may not be "100 per cent up to speed" with current rules as he had just returned to England after three weeks abroad.
"I'm extremely sorry for the slip up and I would urge absolutely everybody to do everything they can to make sure they do follow the rules about masks and social distancing," he said.
"The fact this was my first day back in the UK after three weeks abroad is, I am sure, no excuse for not knowing the rules."
Last week, England expanded its face mask rules alongside other new meaures - including a 10pm curfew for pubs and restaurants - to tackle the spread of the virus.
The prime minister announced fines would be doubled for those caught without a face covering where one is required, as well as more situations - including when travelling in taxis or when moving around hospitality venues - where people must wear masks.
Face coverings have been compulsory for customers in shops and people travelling on public transport since 24 July.
During the Downing Street press conference on Wednesday, the British PM implored the public to follow coronavirus guidelines.
"Bear in mind that the fines are now very considerable and they will be imposed," Mr Johnson said.
Additional reporting by Press Association