De Mol has raised his shareholding from 2.9 percent to 3.5 percent, adding further speculation to stock market rumours that United is to be the subject of a full takeover bid.
The Dutchman is understood to have met with United chief executive Peter Kenyon following last month's financial results presentation, telling him he thought the club was undervalued and indicated that he would be buying more shares.
Last month Irish horse racing tycoons JP McManus and John Magnier increased their stake to 10.37 percent making them the club's biggest shareholders, surpassing BSkyB's 9.9 percent.
Dutch media mogul John de Mol - the creator of the Big Brother reality TV franchise - has increased his stake in English soccer club Manchester United.
