Health Secretary Matt Hancock has been seen travelling in his chauffeur-driven car without wearing a mask, against the advice of No 10.
The public face fines of £200 if they fail to wear a covering in taxis or private hire cars.
There is an exemption for chauffeur-driven cars, but Downing Street said it had advised all its ministers to wear coverings.
A No 10 spokesman said there were masks available in all ministerial cars.
The picture was first published on the Daily Mirror website.
It shows the health secretary arriving at the Department for Health and Social Care on Monday without a mask.
The BBC understands Mr Hancock had been wearing a mask on the journey, but removed it as his car approached the department.
Asked later whether the minister would be reprimanded for going against the advice, the prime minister’s official spokesman said he had not seen the photo.
He added: “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.”