The accountancy watchdog is debating whether to launch a probe into the auditors of the Southeastern train line after the company last week admitted to hiding £25m owed to the state.
The Financial Reporting Council is looking into whether audit giants EY and Deloitte have a case to answer, sources told the Sunday Telegraph.
Last week Southeastern’s finance boss Elodie Brian departed and the operator’s majority owners Go-Ahead’s shares slumped after it said it had failed to declare and repay £25m to the taxman.
EY signed off the transport firms account up until 2016, when Deloitte took on the job.
The transport firm is thought to have referred itself to the Serious Fraud Office over the matter, the report added.
On Friday Greenwich Council’s leader said Transport for London was ready to carve out Southeastern’s metropolitan London services and run them.