Sports Direct International has almost doubled its 15% stake in Umbro, throwing into doubt Nike's plans to buy the maker of the England football kit.
Nike could now struggle to secure its £285m purchase of Umbro
The share swoop took the UK sportswear firm's Umbro stake to 29.9% and sent Umbro's shares up as much as 4.61% to an all-time high of 198.75p.
The move allows Sports Direct to block Nike from taking control of Umbro in a £285m deal announced last week.
Umbro's shares closed at 193.5p, above Nike's 193.06p-a-share offer.
The share purchases by Sports Direct, which owns London-based Lillywhites and the Sports World chain, follows rival retailer JJB Sports move to buy a 10.1% stake in Manchester-based Umbro earlier in the month.
Both Sports Direct and JJB are major customers of Umbro, which also supplies the kit for six English Premier League clubs and football clubs abroad, including Portugal's Benfica and Israel's Hapoel Tel Aviv.
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Shares in Sports Direct declined 4.58% to 135.5p at the close of trade in London.
The Umbro board said last week that it had unanimously backed a Nike takeover of the firm, under which its relationship with the FA would be protected.
Umbro chief executive Steve Makin called the offer an "excellent deal" for shareholders.
A successful bid would help Nike, the world's largest maker of athletic footwear, to boost its presence in the lucrative football market and compete better with German rival Adidas ahead of the 2010 football World Cup.
But in order for it to succeed, US-based Nike requires 75% of Umbro shareholders to approve the transaction.
Umbro has tried to reassure retailers that they should not be concerned by the change in ownership, but the recent share buying suggests that they are not appeased and want to protect their stake in the market for England football shirts.
Sports Direct is controlled by UK billionaire businessman Mike Ashley, who bought Newcastle United earlier this year.