Global accounting firm Deloitte's audit of FTSE 250 professional services company Mitie is under investigation.
Confirming the move on Monday (31 July), the Financial Reporting Council (FRC) said its investigation pertains to Mitie's financial statements for the years ending 31 March 2015 and 2016.
In June, Mitie posted a full-year operating loss of £42.9m for the year to March, dragged down by one-off costs of £88.3m.
The company's accounting review, which was carried out by KPMG, found Mitie's accounting practices to be "less conservative" than its peers.
Responding to the FRC probe, Deloitte said it took the investigation "very seriously" and would co-operate fully.
"Audit quality is of critical importance to our firm and we are committed to maintaining the highest professional standards. We take this investigation very seriously and will co-operate fully with the FRC."