Boris Johnson's father has admitted he "slipped up" after being photographed again breaching rules on wearing masks, this time at a London Underground station.
After the photograph was published, showing him at a Bakerloo line station with his mask under his nose, Stanley Johnson insisted he was unaware that rules also applied when standing on the platform.
When informed that doing so was required on all areas of the London Underground, he told the PA news agency: "I'm happy to be informed by you that this is indeed the case... it's only my third day back in London. I am learning every day."
Mr Johnson attracted criticism last week after being pictured without a mask in a shop, despite his son's Government imposing a law requiring their use.
His latest breach was noticed shortly after Mr Johnson posted a picture on Instagram of himself, Pensions Secretary Therese Coffey and Tory MP John Penrose with the caption: "Masks firmly in place on the Jubilee Line."
After the Tube incident, which took place on Monday morning, he said: "I have certainly been absolutely wearing a mask on the Underground trains and public transport and so on. I don't notice that everybody wears masks on the platform.
He added: "I have certainly got my mask on the platform, I agree it is below my nose.
"This has been a fast learning curve for me and I'm certainly grateful for the vigilance of my fellow travellers," he added.
"Thanks to all of you for sending photographs of me every time I appear in public. I am really taking a huge, huge... I'm taking this very seriously," he added.
Downing Street said the coronavirus rules should be followed by everyone.
The Prime Minister's official spokesman said he did not know whether Boris Johnson had spoken to his father about the latest incident.
But referring to previous images of Stanley Johnson without a mask, the spokesman said: "I know the Prime Minister's father has set out that he recognises his error and fully understands it's vital for everyone to abide by the rules on face coverings and social distancing.
"As I said last week, the Prime Minister is very clear that everyone needs to follow the guidance on hands, face, space – the rules apply to all of us."
Asked whether Mr Johnson should face a fine, the spokesman added: "It's not for me to advise the police in relation to individual cases, that's not how it works in this country."
Transport for London's website states: "Face coverings must be worn for the full duration of journeys on the public transport network, including inside stations, and in taxis and private hire vehicles."
Those found breaking the rule could be subject to a £200 fine, which doubles for each breach up to a maximum £6,400.