The Olympic Delivery Authority's bus services contract, which relates to the transport of staff between venues at the games, states that each organisation involved in any consortium bidding for the contract must have an annual turnover of at least £5m.
Currently, only two of the capital's 22 community transport groups - Hackney Community Transport and Ealing Community Transport - would qualify.
Steve Sears, chief executive of Ealing Community Transport, described the £5m figure as an "arbitrary number". He said: "There is a policy at the highest level that social enterprises should be given the opportunity to bid for services, but that's not operational."
Jonathan Bland, chief executive of the Social Enterprise Coalition, said: "There are lots of social enterprises that can provide great services for Britain, and if they are not being used it's lamentable.
"This is indicative of a trend in government to take a narrow view, looking at the cheapest contracts that can be delivered by organisations on a large scale. We need the Government to take a more intelligent view of commissioning."
An ODA spokesman said that the contract followed government guidelines. He added: "The authority intends to procure a number of direct contracts with major companies. These will then be subcontracted to about 50,000 smaller organisations."