The Tory leader should have registered his use of the gym in a luxury apartment block on Albert Embankment, London, the Parliamentary Commissioner for Standards found.
Elizabeth Filkin's ruling, upheld by the Committee on Standards and Privileges, will prove particularly embarrassing for Mr Hague because of its timing. He used the gym for almost two years, but stopped when Lord Archer was forced to withdraw from the elections for London mayor. The millionaire peer was accused of asking a friend to lie for him over his whereabouts after he was accused of sleeping with a prostitute.
Mr Hague wrote to Ms Filkin for advice after learning that a Labour MP, Fraser Kemp, planned to make a formal complaint. He had been using the gym regularly, at Lord Archer's invitation, since January 1998, and had paid for his own tuition and equipment, Mr Hague said. The commissioner replied that he should register the benefit and he did so.
In the letter to Ms Filkin, Mr Hague said: "Having taken the advice of the registrar, I recognise that the use of the gymnasium could be interpreted as a benefit in kind. I have therefore taken action to register it."
Mr Hague, who is a green belt in judo and trains regularly with his aide Sebastian Coe, is now believed to be searching for an alternative gym. Although he has considered several places, he has yet to find a venue that meets his requirements both for privacy and for specialist judo mats.
Membership of a private gym in Westminster costs around pounds 1,000 a year, and Ms Filkin ruled that the free use of such a facility should be listed in the Register of Members' Interests. The rules pointed out that benefits worth more than 0.5 per cent of a member's salary - about pounds 235 - should be registered if it might be thought to relate to membership of the House or if it influenced the member concerned. "I uphold Mr Kemp's complaint that Mr Hague should have registered this benefit as soon as he began to use it regularly. I am satisfied that Mr Hague has acted promptly to correct the omission," she wrote.
Conservative Central Office declined to comment on the committee's report. "As far as we are concerned, the matter is closed," a spokesman said.
Mr Kemp said the case "highlights the Tories' dependence on a very few rich backers".Reuse content