Getty ImagesEY 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.
The FRC said the recent public hearings carried out as a broader inquiry into the Horizon scandal did not cover EY's audits of the Post Office, so will not be covered by its investigation.
More than 900 sub-postmasters were prosecuted for stealing because of incorrect information from the Horizon computer system. It has been called the UK's most widespread miscarriage of justice.
The Post Office itself took many cases to court, prosecuting 700 people between 1999 and 2015.
