The Ministry of Defence is urgently making preparations to call up as many as 2,000 more troops for Olympic security operation if G4S fails to deliver on its revised target of 7,000 staff in place by next week's opening ceremony, Whitehall sources say.

A fresh urgency was injected into the contingency preparations after defence officials started working with G4S in the last few days and gained a better understanding of the problems facing the company. Whitehall sources told the Guardian the contingency involved the possibility of calling up between 500 and 2000 more troops.

The home secretary Theresa May, who only last Wednesday authorised the call-up of 3,500 extra troops to bail out the private security company, has not made any further official request for troops and is working to avoid exactly that situation.