The ongoing HM Revenue & Customs investigation into British football that has so far seen the homes of Newcastle and West Ham raided is said to be focusing on 'secret payments' made to players and agents during the transfer of players between English and French clubs.
Newcastle managing director Lee Charnley was arrested and questioned as part of the investigation on Tuesday night but was ultimately released without charge less than 24 hours later.
French authorities have been working alongside UK HMRC officers on the cases.
A BBC published statement from the French Prosecutor's Office explains, "The British authorities suspect secret payments may have been made to benefit certain players, their agents or third parties, allowing them to avoid paying tax on the income or making social security payments."
An HMRC statement on Wednesday confirmed that locations in north east and south east England had been searched and that business documents, financial records, computers and mobile phones were among items seized by the authorities in the raids that took place.
It was July 2016 when French prosecutors were first brought on board.