Middlesbrough have stunned Derby County by announcing their intention to sue the Championship club, days before the play-off final at Wembley.

Steve Gibson, the Middlesbrough owner, has taken drastic action in his dispute with Mel Morris by vowing to take legal action over what he insists are clear breaches of financial rules.

Gibson is alleging that Morris has broken the English Football League’s profitability and sustainability rules and Middlesbrough officials contacted Derby on Friday to inform them of their stance.

Boro are understood to be furious that Derby reported a £14.6m profit in their 2017/18 accounts, after Morris sold the club’s Pride Park stadium and then leased it back. Gibson believes that Morris has flouted the rules.

Derby are adamant they have been fully compliant, and the timing of Middlesbrough’s shock move has surprised the club ahead of their play-off final against Aston Villa on Monday.