Taxpayers could be faced with a £1bn bill following the collapse of the Tube maintenance firm Metronet, an independent arbiter has said.
Transport for London says it wants to take over Metronet's contracts.
Chris Bolt said the consortium could be owed up to £470m for work over and above that specified in its contract.
Mr Bolt, a PPP arbiter, said it could also be due between £230m and £600m to cover cost increases.
Metronet was upgrading nine London Underground lines before going into administration in July.
Mr Bolt's views came in what he described as his "initial thoughts" on the financial situation regarding Metronet. His final thoughts will come later in the year.
Transport for London has said it wants to take over the Metronet contracts.