West Ham manager Sam Allardyce is taking legal action against former club Blackburn and successor Steve Kean.
Full details of the High Court case have not been made public.
Allardyce reportedly instructed solicitors after a video clip was published on the internet in May which allegedly showed Kean criticising him.
A spokesman for Allardyce said: "He has issued legal proceedings against Steve Kean and Blackburn Rovers but is unable to comment further at this time."
No-one from Blackburn was available for comment.
The case was filed at the High Court in London on 12 July.
Kean, who replaced Allardyce as Blackburn boss in December 2010, has stayed on at Ewood Park despite the club's relegation from the Premier League last season and calls from many supporters for him to be sacked.
In the video, Kean appears to make unguarded remarks about Allardyce to supporters in a bar during the club's pre-season tour of Hong Kong in 2011.