Chancellor George Osborne did not realise a car he was travelling in was parked in a disabled bay at a motorway service station, the BBC understands
The Daily Mirror published a photograph of the chancellor getting into a Land Rover parked in the marked bay at Magor services on the M4 near Newport.
But a Treasury source said Mr Osborne had been dropped off to buy lunch.
The unmarked police Land Rover was not driven by the chancellor at any point, added the source.
The incident is understood to have taken place on Wednesday as the chancellor returned from a speaking engagement at a nursery in Cardiff.
The Treasury source told the BBC the chancellor had got into the car and left the scene without realising that it had been parked in the bay.
Mr Osborne did not condone his driver's parking decision, the source added.
But a witness reportedly told the Mirror: "I couldn't believe my eyes when I saw his driver pull into the disabled parking bay.
"It's not as if they were in a rush to move. The car park wasn't exactly full and there were plenty of other, ordinary spaces."