Buckles admitted he was “extremely disappointed” with the way the problems were managed, and stressed that G4S’s plans for the Paralympics were in place.
He said the firm could “resource it fully with no external interventions”.
Buckles said he was confident the security firm would “over deliver” on the contract for the Paralympic Games, which begin tonight.
In July, G4S admitted it could not recruit the necessary 10,400 security staff for the Games, and an extra 4,700 military troops had to be called in to make up the shortfall in G4S staff as a result.
The findings of a review by PwC into the failings of the private security group are expected in late September.
In its half-yearly report, G4S said that £3.8bn of work was in the pipeline, including new contracts with two prisons and a police force.
It posted flat profit year on year of £236m, with revenue up 5.8 per cent to £3.9bn.
Emerging markets, which benefited from growth of 12.7 per cent over the first half, performed well over the period. The interim dividend was held at 3.42p.
G4S is undergoing a restructuring to trim costs by around £30m over the next year, including cutting around 1,100 jobs.
It also confirmed yesterday it had no plans to bid for contracts relating to the upcoming World Cup or Olympics in Brazil.
Shares closed down 1.95 per cent.