The Bill & Melinda Gates Foundation Trust and Cascade Investment, an asset management firm owned by Bill Gates - one of the world's richest people - increased their combined holding in G4S to 3.2 percent last week by acquiring around 6 million more shares, G4S said on Monday.
G4S, which runs services such as cash transportation and prison management in over 125 countries, suffered a blow to its reputation when it failed to provide a promised 10,400 guards for the London Games. Following a profit warning in May, its chief executive stepped down last month.
Gates's investments range from stakes in the Canadian National Railway Co. to global drinks group Diageo and British carpet firm Carpetright.
The Bill and Melinda Gates Foundation, founded by Gates and his wife in 2000, works to eradicate the world's most deadly diseases and poverty.
Shares in G4S, which have fallen 18 percent in three months, were down 1.2 percent at 241.55 pence by 12:21 p.m. British Time on Monday.
(Reporting by Neil Maidment; editing by Tom Pfeiffer)