All Government departments, including the Foreign Office, the Home Office and the Ministry of Defence, have been told to "market test" security arrangements.
Private companies will also be asked to bid to make and install alarm systems and detection equipment - a role previously carried out by secretive Government security advisers.
Group 4 and the security division of Rentokil, the pest control firm, say that they would be interested in pursuing the Government guarding contracts.
Securicor, the other leading security guard company which also runs private prisons, is likely to be a leading contender for the Government work too.
But the prospect of private security guards protecting ministers' offices has alarmed some MPs, who fear that hiring unlicensed personnel could leave a dent in the Government's security cordon. "I have general concerns about private sector guarding in that I think it is a wholly unregulated industry," said Alan Beith, Liberal Democrat home affairs spokesman. "Government departments should not hire private security staff without carrying out thorough security checks."
The prestigious Whitehall security corps, many of whom are ex-servicemen and women, vet all visitors to buildings and make sure they are not carrying guns or explosives or do not wander unauthorised around the buildings.
Sensitive documents relating to the defence of Britain and market-sensitive trade decisions are kept in the Whitehall vaults. It is also where ministers work through their red boxes and take advice from civil servants.
"In the past, the guards have been hired through an agency of the Cabinet Office, but now if a Government department hires security employees they are market tested. They will look at value for money," said a Cabinet Office spokesman.
Only bomb protection advice, and the development of specialist security equipment, will remain wholly in Government hands.