Jeremy Hunt sought to reassure Olympic ticket holders from around the world today that the Games would be safe and secure.
The Culture Secretary admitted organisers were "let down very badly" by G4S, and said contingency plans were in place for the possibility of further staff recruitment problems.
When asked at an Olympic Park press conference if yesterday's bomb attack on a bus full of Israeli holidaymakers in Bulgaria presented extra security challenges to the Games, he replied: "Obviously we're monitoring the whole time what's happening with respect to the changing security situation.
"We have extremely competent intelligence services who are giving us advice and we're responding to that on an ongoing basis.
"But the world can be absolutely certain that we will deliver a safe and secure Olympics. It's always been our number one priority."
Mr Hunt said the failure of G4S to provide enough staff had not damaged the integrity of security plans.
"It's a huge project doing an Olympic Games - the biggest sporting event on the planet - and from time to time you do have a contractor who lets you down.
"We were let down very badly but we always had a robust back up plan in place which we are now activating and we're continuing to monitor the situation on a daily basis.
"This has meant that the mix of venue security guards has slightly changed - more armed services, more police, fewer private sector contractors - but the integrity of the plans remain absolutely intact."
The minister was also questioned about the potential for more troops being used at the Games.
He replied: "We do have other contingency plans in place which we would activate at any time if we felt there was likely to be a further shortfall."