Middlesbrough expressed frustration after being blocked from joining disciplinary proceedings launched by the English Football League against Southampton, saying they hold “relevant factual evidence” about the alleged spying incident.

The final, dubbed the “richest game in football” because of the huge financial rewards of playing in England’s top flight, will be played at Wembley Stadium.

Southampton were charged by the EFL last week after Middlesbrough complained of unauthorised filming at their training ground.

Southampton CEO Phil Parsons said on Tuesday that they were fully cooperating while also undertaking an internal review to ensure that all facts and context were properly understood.

Southampton will play Hull City for a place in the Premier League on May 23 after beating Middlesbrough 2-1 in their Championship playoff semi-final second leg.

“The conduct at issue, namely the observation and recording of our training session ahead of a fixture of such significance, goes to the heart of sporting integrity and fair competition,” Middlesbrough said in a statement.

“In these circumstances, the only appropriate response is a sporting sanction which would prevent Southampton FC from participating in the EFL Championship playoff final.

“We remain hopeful that the EFL, as regulator, will pursue such a sanction before the Disciplinary Commission in order to protect the integrity of the game, safeguard all member clubs, and deter any attempt in the future to obtain an unfair and unlawful advantage in pursuit of promotion to the Premier League.”

Middlesbrough also said they would take legal action if needed.