Stobart Group has sacked former boss Andrew Tinkler from the board in the latest twist in a bitter power struggle at the owner of Southend and Carlislie airports.
The firm is suing Mr Tinkler to court for “breach of contract and breach of fiduciary duty”, it said.
Mr Tinkler claimed the board had defamed him as part of a legal dispute with Stobart Group.
He is seeking to remove chairman Iain Ferguson in a vote at the firm’s AGM.
Mr Tinkler was chief executive of Stobart Group for a decade until he stepped aside last year to focus on a firm called Stobart Capital, but remained on the Stobart board.
The dispute between Mr Tinkler and Mr Ferguson erupted after Mr Tinkler came up with a proposal late last year to buy the airline Flybe.
Relations deteriorated between Mr Tinkler and the firm last month when he told the board he planned to oppose the re-election of Mr Ferguson as chairman at Stobart Group’s annual meeting on 28 June.
Mr Tinkler, former director Allan Jenkinson, and Neil Woodfood of Woodford Investment Management have banded together to try to oust Mr Ferguson as chairman. Together, they control about a third of the shares in the company.
They are seeking to replace Mr Ferguson with Philip Day, chief executive of Edinburgh Woollen Mill Group.
However, Invesco, which has about a 24% holding, has declined to support the move.
The firm responded on 29 May by advising investors to re-elect Mr Ferguson. Mr Tinkler has alleged this letter was defamatory and is suing five directors for defamation.
In recent weeks Stobart Group has begun pursuing Mr Tinkler in the courts for a tax liability it says is worth £3.8m plus interest.
This week the law firm Rosenblatt sent Mr Tinkler’s lawyers a letter that has been seen by the BBC. The letter alleges that he tried to profit from the proposed Flybe deal in a way that would have been detrimental to Stobart Group.
The letter also alleges that Mr Tinkler has taken “numerous and substantial” expenses out of the company.
Mr Tinkler said the allegations in the Rosenblatt letter contained “malicious falsehoods” and that he will “rigorously defend himself against the chairman and chief executive’s attempts to defame him”.
Stobart Group, which is worth more than £800m, is separate from the road transport firm Eddie Stobart Logistics. That firm is also listed on the London market and is worth just over £500m.