Boro - who missed out on a Championship play-off place by finishing one point behind Derby last season - have sent a legal letter to the EFL, whose board met on Thursday.

The EFL are set to conduct independent valuations regarding sales by Derby, Sheffield Wednesday and Reading of their own grounds to their owners.

Both Middlesbrough and the EFL have declined to comment on the issue, first reported by The Times.

In May, the issue was raised by Leeds United owner Andrea Radrizzani, who accused Derby of "cheating" after owner Mel Morris bought Pride Park for £80m and leased it back to the club, helping the Rams achieve a profit of £14.6m.

Earlier this summer, it was reported some Premier League clubs wanted the apparent loophole closed.

Derby have defended the sale of their stadium, saying they conducted an independent commercial valuation prior to the deal being completed.

Aston Villa's sale of Villa Park to the club's owners is also set to be scrutinised by the Premier League, to establish whether it may have breached financial fair play rules.

The stadium was sold in May for £56.7m to owners Nassef Sawiris and Wes Edens via NSWE Stadium Ltd, a company controlled by the two billionaires.

The deal helped Villa comply with the EFL's profit and sustainability rules.

Villa, promoted to the top flight in May, insist any such review is standard practice, and part of the normal assessments that all clubs face.