The Rugby Football League is ­facing a potential revolt after a number of high-profile professional rugby league coaches signed a letter calling for the Sheffield coach, Mark Aston, to be allowed an independent appeal against his 18-month ban for a breach of head injury protocol.

Aston, who has been involved with the Eagles for almost 40 years as player and coach and was man of the match in their 1998 Challenge Cup final win against Wigan, has been told he will be suspended until April 2026 after a tribunal determined he “deliberately flouted the rules” on head injuries earlier this year.

Aston selected the Sheffield half-back Matty Marsh in their cup tie against Wigan in April despite the player not being signed off by the club’s medic, Hannah Cole, as she felt the return-to-play protocol had not been followed correctly.

The coach insists he had been informed on the morning of the game that Marsh was available to play but the correct paperwork was not filed and sent to the Sheffield coach informing him of the player’s unavailability. The Eagles’ physio, Mick Heys, contacted the RFL on the day of the game to indicate Marsh had passed the relevant tests.

However, Cole did not sign Marsh off, leading to what the tribunal ­suggested was a serious breach of protocol but what Aston’s ­supporters maintain was an “administrative error in paperwork completion”. Aston says he was unaware of Marsh’s unavailability.

But, the tribunal heard, he was not copied into relevant emails over Marsh’s situation, with those emails not disclosed in the tribunal due to “legal privileges”. Aston almost immediately signalled his ­intention to appeal against the ban but a request for it to be heard by the independent body, Sport Resolutions, was refused by the RFL.

Aston has until Monday 18 November to ­officially lodge an appeal but there are growing calls demanding it is heard independently rather than through an RFL-appointed panel. The Guardian understands that 15 active coaches have signed a letter, which has been written by Sheffield’s legendary former coach John Kear, who masterminded their 1998 Challenge Cup triumph, calling for an independent hearing.

That list is believed to include ­several Super League coaches as well as individuals in France and Australia, and Aston’s supporters hope the calibre of people involved will convince the governing body to allow Aston the chance to have his appeal heard by Sport Resolutions.

The RFL has been approached for comment.