Sydney Airport Corporation, the owner of the city's airport at Mascot, is unlikely to participate in the construction or running of the new airport at Badgerys Creek, a global ratings agency says.

If the ratings agency is proved right, that would mean the federal government would have to build the airport itself or contract with a rival to Sydney's existing airport operator.

The likelihood of Sydney Airport also operating the Badgerys Creek airport took a hit late last year when the Turnbull government said it would not make extra funds available for doing so.

The government had intimated to Sydney Airport, which owns a contractual right to be given the first chance to build and run a Badgerys airport, that it would provide billions in concessional loans.