Bill Gates
Bill Gates has bought six million more shares in UK security firm G4S (Reuters)

Microsoft's billionaire founder Bill Gates has increased his stake in UK-based security giant G4S, after new boss Ashley Almanza outlined plans to turn around the troubled firm.

A regulatory filing shows that both the Bill & Melinda Gates Foundation Trust and Cascade Investment, also owned by Bill Gates, have increased their joint stake in G4S to 3.2% by buying six million more shares - and are now holding a total of 45,224,081.

G4S has been rocked by a number of setbacks in recent years, including an ominous profit warning, a costly mishandling of its Olympics security staffing contract, and a botched takeover attempt of cleaning contractor ISS.

Almanza hinted at a round of job cuts, by pledging to cut costs and to shift focus to emerging markets, under his new strategy.

In the first quarter, G4S operating profit margin slipped 0.6% on the same period a year before. It blamed ongoing economic woes in its European market and pricing pressures in the UK in the unscheduled warning ahead of its full year results.

Around £600m (€706m, $934m) was knocked off its market value, after it sounded the siren, but the firm's annual results for 2013 reported profit rising 6% to £516m.

G4S also took an £88m hit from its London 2012 Olympics staffing gaffe, when it was forced to admit it had not been able to find and train enough security personnel to meet its contract with the games' organisers. The military and police force had to plug the security gap.

"Let me reassure you that while the short term outlook is challenging the underlying business and the medium and long term outlook remain very strong indeed," said Almanza at the firm's annual shareholders' meeting.

"We are putting in place a number of business improvement plans to strengthen margins in 2014 and 2015."