Health Secretary Matt Hancock has been pictured in his ministerial car with no face mask on.
Passengers of chauffeur-driven cars are exempt from the £200 fine that can be imposed on the public if they fail to wear a face covering in taxis or private hire cars.
However, Downing Street said it has advised all ministers to wear face coverings.
Mr Hancock was arriving at the Department for Health and Social Care when he was seen without wearing a mask.
He was reportedly wearing a face covering for part of the journey, but took it off as he neared the department.
Boris Johnson’s spokesman said he had not seen the picture, but that “on the general point, we set out at the time that we were making face coverings available in all ministerial cars so that ministers would be able to wear them”.
Last month, the PM’s spokesman said that Mr Johnson and ministers would be covering their faces in line with the guidance.
The fine for not wearing face coverings in shops, supermarkets, public transport, taxis and in indoor hospitality recently doubled to £200 for a first-time offence.
The rules apply also to customers inside restaurants and pubs when they are not seated at a table.