The share ownership was revealed when Mordashov's investment vehicle, S-Group Capital Management, increased its stake past 15 percent, one of the standard disclosure thresholds under German law.
"Officially, this makes S-Group Capital Management the largest single shareholder in TUI," Robin Zimmermann, a spokesman for the German firm, said by telephone Monday.
S-Group's stake in TUI passed the 10 percent mark in April.
Mordashov has backed TUI's managers in their plans to sell off TUI's Hapag Lloyd container shipping division to focus on the firm's tourism business. Norwegian shipping tycoon John Fredriksen, who held 15.01 percent of TUI stock as of July 2, has opposed the sale of Hapag Lloyd.
In May, Fredriksen reportedly offered Mordashov $700 million for his then-10 percent stake, which had a market value of $470 million. Mordashov refused the offer.
TUI is currently proceeding with plans to sell Hapag Lloyd. First-phase bids were due Monday, and Zimmermann said TUI hoped to reach a final decision regarding the sale by October.
"We see high growth potential in the tourism industry worldwide and especially in emerging markets such as Russia," S-Group Capital managing partner Vladimir Yakushev said in a statement Monday. "Our stake increase also reflects our conviction that [TUI] is ideally positioned to capture this potential."
No changes were expected on the TUI supervisory board as a result of S-Group's latest shares purchase, Zimmerman said. Yakushev already represents S-Group on the board.
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