David Cameron's father-in-law is among rich landowners cashing in on Labour's green subsidies, with a wind farm generating an estimated £3.5million a year on his country estate.
Sir Reginald Sheffield, 63, who is worth at least £20million, splits the profits with the project's developers.
Around half of the income comes from a government scheme to make power companies use more renewable energy, much of it bought from private generators. It is subsidised by every household, via their electricity bills.
Sir Reginald's eight 400ft turbines were switched on last August at Bagmoor, part of the 3,000-acre Normanby Hall estate near Scunthorpe that has been in his family since the 16th century.
He plans a second development at nearby Flixborough Grange, despite fierce opposition from locals.
Samantha Cameron, 38, is the elder of his two daughters by his first marriage to Annabel Jones, who later married former Tory minister Lord Astor. Her half-brother Robert, 24, helps run the family's Normanby Estate Company.