Accounting giant EY is being investigated over its audits of the Post Office in relation to the Horizon scandal from March 2015 to March 2018.
The Financial Reporting Council (FRC), the UK's accounting regulator, said it will check whether EY met its standards "with particular reference to matters related to the Horizon IT system".
The software developed by Fujitsu was at the heart of the Post Office scandal that saw hundreds of postmasters wrongly convicted.
EY said it takes its responsibilities "extremely seriously" and will be "fully cooperating" with the FRC.