Microsoft creator turned philanthropist Bill Gates has upped his stake in troubled security giant G4S.

The Bill & Melinda Gates Foundation Trust and Cascade Investment, an asset management firm owned by Mr Gates, increased their combined holding in G4S to 3.2% last week by acquiring about six million more shares.

G4S, which operates in over 125 countries, suffered a blow to its reputation when it failed to provide a promised 10,400 guards for the London Olympics.

A profit warning last month triggered the departure of chief executive Nick Buckles, who was succeeded on June 1 by Ashley Almanza.

The Gates trust was set up in 2000 to fight poverty and disease. Its other UK investments include Diageo and Carpetright. Shares in G4S, which have fallen 18% in three months, hit an 11-month low yesterday closing down 1.8p at 242.8p.