Birmingham have promised to co-operate fully with police after being raided by officers who took away financial documents early on Wednesday morning.
There is no suggestion Birmingham have done anything illegal and the Barclays Premier League club are giving their full support to police.
Managing director Karren Brady told The Sun online: "We are co-operating fully with the police in their investigation, which is in connection with a third party."
Birmingham, in a statement, said: "The club is fully co-operating with the police in their inquiries which relate to an unconnected third party or third parties.
"For the avoidance of doubt no-one connected with the club has been questioned or arrested. No further comment will be made by the club."
A Police spokeswoman said: "We can confirm that officers from the City of London Police executed a search warrant at a premises this morning in connection with the ongoing investigation into football corruption."
