Sir Ken Jackson, once the Prime Minister's favourite trade unionist, made £220,000 profit on property transactions funded by his union, an internal document revealed.
The surplus was pocketed by Sir Ken on the sale of a union-financed house and flat as part of a golden handshake when he was voted out of office.
A memorandum prepared for senior officials at the Amicus union said Sir Ken "managed to exploit the property inflation cycle to quite an extraordinary degree in that he bought at the bottom of the market and sold at the top".
The union bought a house for him in Bromley, Kent, for £280,000 around six years ago and sold it for £480,000 with the difference going to Sir Ken. It then funded the purchase of a flat for some £100,000, which was later sold for about £120,000. The union is expected today to place a statement on its website abolishing the perk for officials currently in office.
Although Amicus will provide housing for a newly elected general secretary if they have to relocate to London, the union "will retain the capital gain from the sale of the property" after the official's term of office comes to an end.
Sir Ken also received a cash payment of nearly £100,000 and a pension worth two-thirds of his £78,000 salary.
During his campaign to retain the leadership of the engineering section of Amicus, some of his supporters were found to have voted more than once during a nominating process that preceded the poll. The left-winger Derek Simpson won the ballot.
Sir Ken said the union had not bought any property on his behalf although it had supplied a loan that partly financed the purchase. He said he benefited from the increase in property prices but it was "not remotely like" the figure quoted.
The union recently came under fire for buying a £600,000 house in Berkhamsted, Hertfordshire, for Mr Simpson. He is being charged a "peppercorn" rent for it, but will have to pay the market rate if he stays there after retirement. Mr Simpson, 58, whose family home is in Sheffield, said: "Because of the ridiculous cost of accommodation in the London area, it is understandable that the union needs to give some assistance. But the capital gains that accrue rightly belong to the union."
Join our new commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies