He worked with Marc Rich, the disgraced commodities trader, and has maintained his connections with the energy industry. He declares that he still owns Harcourt Consultants, which advises on oil and gas matters. Last year, it emerged that Mr Duncan’s private office was being funded by donations from Ian Taylor, the chairman of Vitol, a controversial oil company. Mr Duncan was shadow business secretary with responsibility for energy policy at the time. He later declared the funding in the register of MPs’ interests.

In the 1990s, Vitol paid $1 million to Arkan, the Serbian war criminal, to act as a “fixer” on a deal in Slobodan Milosevic’s Serbia that had collapsed. In 2007, the company was fined over the oil-for-food scandal. Vitol pleaded guilty to larceny in a New York court and paid $13million to the Iraqi people in restitution.

Mr Duncan’s main home in Westminster has also attracted controversy in the past.

Mr Duncan bought his first house on the street in 1986 and lent it to John Major as the base for his leadership campaign. In 1992, Mr Duncan was elected and was soon made a minister under John Major.Within weeks it emerged that he had lent his elderly next door neighbour money so that he could buy his home under the right-to-buy legislation.

The neighbour bought the 18th century council house at a significant discount and sold it to Mr Duncan just over three years later. In the ensuing furore, Mr Duncan resigned from his ministerial position.
He then combined the two properties into one. Mr Duncan said yesterday: “It was me who raised the issue of gardening costs with the fees office. “Although it was a legitimate claim, we agreed that it might be seen as too large a single item and therefore I did not claim it.”