The move, from a previous holding of around 10 percent, put Mordashov roughly on par with Norwegian investor John Fredriksen and underscored a battle over how the company should develop.
Mordashov backs the strategy of TUI boss Michael Frenzel, while Fredriksen has led a shareholder revolt against the company's management and bid earlier this month for Mordashov's 10-percent stake to gain a decisive holding in TUI. Frenzel wants to sell the group's Hapag-Lloyd shipping unit to concentrate on tourism, where TUI is a European leader.
Some press reports have said TUI was exploring a deal with Singapore's Neptune Orient Lines, which was said to be considering an offer of as much as €6 billion to €7 billion ($9.5 billion to $11 billion) for Hapag-Lloyd, the world's fifth biggest shipping line.
Fredriksen, who made a fortune in the container shipping sector, challenges such a move and demands that shareholders be consulted on the matter. The Norwegian has proposed that each activity be run separately, with investors able to decide whether to back one or both units.
He has also called for Frenzel's resignation, noting that TUI has lost much of its value during his 14-year reign.
Mordashov has express firm support for Frenzel, and has also said he wants to launch a joint venture with TUI on the fast-growing Russian tourism market.