Mittal spent eight years at the top of the Sunday Times rich list but has dropped to fourth place in this year's survey, after his estimated fortune fell by £2.7bn to £10bn.
The 40% held by Mittal and his wife, Usha, stake in steelmaker ArcelorMittal has dropped as the value of the company's shares fell from more than €12 (£10.30) a year ago to €9, hit by the global economic downturn and cheap competion.
But Usmanov moved from second on the list to the top spot after the value of his assets rose by £985m to £13.3bn. Russia's richest man, whose business empire started with the manufacture of plastic bags but now encompasses iron ore producer Metalloinvest, a major stake in mobile phone operator MegaFon and a share in internet group mail.ru, Usmanov also owns Sutton Place, the former Surrey home of the late oil baron J Paul Getty and a £48m London mansion.
He made £1.6bn last year by selling Facebook shares when the social media group listed on the stock market.
The annual rich list is dominated by Russian and Indian billionaires, with the highest ranking Briton being the Duke of Westminster, worth £7.8bn thanks to his property holdings in London's Mayfair and Belgravia and ranked eighth.
The biggest riser was Russia's Len Blavatnik, the owner of Warner Music, whose wealth climbed by £3.4bn to £11bn after an increase in the value of his various shareholdings. Last month, he received £2bn for his stake in TNK-BP.
Belying the tough economic conditions in the UK, the number of billionaires climbed to 88 from 77 last year. Since the list was first published in 1989, the combined wealth of the top 200 has jumped from £38bn to £318.2bn.
In the first list, the richest person was the Queen with total wealth of £5.2bn. But this included the crown estates and the royal art collection, which have subsequently been excluded from the calculations.
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