Swansea City are suing former boss Russell Martin for breach of contract following his move to Southampton in June 2023.
Martin, 38, left Swansea after two years in charge to take over at St Mary's.
Swansea have declined to comment but have confirmed the legal action against their ex-head coach.
Martin's move to Southampton was held up because of a dispute over compensation due to Swansea.
With that dispute still unresolved, Swansea have opted to sue - though they are not taking action against Southampton.
Martin, who left MK Dons to take charge at Swansea in 2021, signed a three-year deal when he arrived in Wales, but left after two seasons in charge.