West Ham and Newcastle football clubs have been raided by police this morning as part of an HMRC tax fraud investigation spanning England and France.
Several people have been arrested in connection with the probe, including Newcastle managing director Lee Charnley, according to reports.
HMRC said it had deployed 180 officers across the UK and France today in connection with an investigation into a suspected £5m income tax and national insurance fraud within the professional football industry.
The BBC has reported around 50 HMRC officers raided West Ham's offices at the London Stadium at 8am today.
"Investigators have searched a number of premises in the North East and South East of England and arrested the men and also seized business records, financial records, computers and mobile phones," HMRC confirmed.
"The French authorities are assisting the UK investigation, have made arrests and several locations have been searched in France."
HMRC added: "This criminal investigation sends a clear message that, whoever you are, if you commit tax fraud you can expect to face the consequences.
"As this is an ongoing investigation HMRC is unable to provide and further detail at this time."
Both West Ham and Newcastle have strong links with the business world.
Newcastle is owned by Sports Direct boss Mike Ashley, and businessman David Gold - father of Ann Summers founder Jacqueline Gold - co-chairs West Ham, while Karren Brady serves as vice-chair.
West Ham have said that they are "co-operating fully with HMRC to assist their enquiries". Newcastle have been contacted for comment.
The raids came just 36 hours after Newcastle secured their return to the Premier League next season with a 4-1 win over Preston on Monday.
They are second in the Championship, having been relegated last season, and could yet overtake leaders Brighton to claim the second-tier title.
West Ham are 14th in the Premier League with four games of the season remaining, meaning they are all but certain to retain their top-flight status.
They have endured a turbulent first campaign at the London Stadium, having relocated from Upton Park, their home of more than a century, to the former Olympic Stadium last summer.
Crowd trouble during their early games at the new venue prompted calls for matches to be played behind closed doors, while the team has struggled to match last season's results, leading to pressure on manager Slaven Bilic.