Yet more pressure has been heaped on the London 2012 Olympics’ embattled security contractor G4S after one of their staff was accused of spitting and hurling verbal abuse at a serviceman drafted in to cover the gaps at the Games.

The security guard is alleged to have called the soldier a “baby-killer”, in an apparent reference to his service in Afghanistan after the member of 35 Engineers asked to search him. The incident is thought to have happened at Lords cricket ground on Saturday, which was hosting the archery event.

G4S said it was “urgently investigating claims of an altercation between a security officer and a soldier”. A spokesman said: “We take any case of improper behaviour by a member of our staff very seriously. We do not tolerate insulting behaviour and, where necessary, appropriate disciplinary action will be taken.”

The Ministry of Defence said the alleged incident was an “isolated occurrence”. A spokesman said: “We can confirm that there was an incident at Lord’s cricket ground on 28 July. The matter was reported and is being investigated by G4S.

“This isolated occurrence is not indicative of the good working relationship between the military and G4S.”

G4S has been widely criticised after it was forced to admit it could not provide all of the security guards it was contracted to deploy at the Games. Its chief executive was dragged before MPs to explain the situation and he agreed that the episode was a “humiliating shambles”, saying he regretted taking the contract in the first place.

It emerged G4S banned its own executives from taking up corporate hospitality places during the Games after the Army was called upon to provide 4,700 personnel to cover the gaps it left in the Olympics security plan, bringing the total number of MoD personnel involved in the Games to 18,000.