Mr Mandelson's aides confirmed the former secretary of state for trade and industry had taken free flights on the private jet of Linda Wachner, the head of Warnaco, an American clothing conglomerate.
Mr Mandelson's spokesman said the trips were made in "a personal capacity". "It was nothing to do with his work at the DTI. That's the reason it was not disclosed in the Register of Members' Interests, which only requires declarations relating to your work as an MP. This trip was paid for by her, out of her own personal bank account, not her company, and that was nothing to do with the DTI."
The shadow Chancellor, Francis Maude, said the Tories would ask the Parliamentary Commissioner for Standards to investigate.