Footage obtained by The Independent shows a maskless Stanley Johnson on board a Tube train as it pulled out of Victoria station on Monday afternoon.
The individual who captured the moment said Mr Johnson had not been wearing his mask while traveling through the station corridor towards the platform and only appeared to put on his face covering after the individual started filming.
The 80-year-old’s breach came just as health secretary Matt Hancock was announcing that London would be moving into tier 3 level restrictions this week due to a surge in Covid-19 cases in the capital.
Mr Johnson was pictured with his face unconvered on another Tube train in October, when he admitted he had “slipped up” and was “learning every day”. He has also been caught on camera failing to wear a mask in a London shop and a Greek airport.
Face coverings have been mandatory on public transport since July under rules announced by the government to slow the spread of coronavirus.
Under those rules, face coverings must be worn for the full duration of journeys on the public transport network, including inside stations.
The latest apparent breach by the prime minister’s father was filmed by a passenger on a Victoria line train at 3.50pm on Monday.
“Stanley Johnson and I were walking the corridor at the same time towards the platforms. I recognised him immediately and that he wasn’t wearing a mask,” the passenger, who asked to remain anonymous, told The Independent.
“He later entered the carriage I was in. I saw him walking towards me and the open carriage still not wearing a mask, so I decided to capture him on video.
He added: “I think he saw me with my phone in my hand and decided then to put his mask on.”
The passenger said they were “annoyed at mask dodgers.”
In a statement sent to The Independent, a No 10 spokesperson said: “The prime minister has been clear that everyone must follow the guidance on hands face space to limit the transmission of the virus.”
The Independent has approached Stanley Johnson’s agent for comment.
In October, Boris Johnson’s father was spotted breaching coronavirus rules three times in a single week.
In one incident, he was seen at an Underground station reading a newspaper with his mask lowered.
In another, he was caught sitting in an airport lounge with his face covering lowered again and in another, he was seen shopping without a mask.
The prime minister’s father has apologised for his past transgressions, asserting back in October that he was still “learning every day”.
“I have certainly been absolutely wearing a mask on the Underground trains and public transport and so on,” he said at the time.