Southampton say the decision to expel them from the Championship playoffs over the 'Spygate' scandal is "manifestly disproportionate" to any other sanction handed down in the history of the English game.
An independent commission imposed the penalty -- and docked four points for next season -- after the club admitted three spying charges, including one related to observing a training session of play-off semi-final opponents Middlesbrough earlier this month.
The commission also reinstated Boro, denying Southampton the chance of a shot at promotion to the Premier League worth an estimated £200 million ($267.9m) at a minimum.
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Saints chief executive Phil Parsons confirmed they were appealing against the sanctions and, while he apologised to supporters for the conduct of club staff, the club are adamant the penalty is far too harsh.
"The commission was entitled to impose a sanction. It was not, we will argue, entitled to impose one that is manifestly disproportionate to every previous sanction in the history of the English game," Parsons said.
"We believe the financial consequence of yesterday's ruling makes it, by a very considerable distance, the largest penalty ever imposed on an English football club."
A league arbitration panel will hear Southampton's appeal on Wednesday afternoon.
Parsons said the club had been "denied the opportunity to compete in a game worth more than £200m and one which means so much to our staff, players and supporters."
He added Leeds had been fined £200,000 for a similar offence, and added: "Luton Town's 30-point deduction in 2008-09 -- to date the most severe sporting sanction in the English game -- was levied against a club already in League Two, with no comparable revenue at stake.
"Derby County's 21-point deduction in 2021 cost them their Championship status. Everton's eventual six-point deduction in 2023-24 followed losses of £124.5m, a figure dwarfed by what has been taken from Southampton in a single afternoon.
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"The largest financial penalty ever levied by the Premier League, against Chelsea in March of this year, was £10.75m, and was accompanied by no sporting sanction whatsoever despite involving £47.5m in undisclosed payments over seven years.
"We say this not to minimise what occurred at this club, which we have accepted was wrong. We say it because proportionality is itself a principle of natural justice."
There is no guarantee that Southampton would have won the playoff final against Hull and be denied the riches associated with earning promotion to the Premier League as a result.
Parsons said what the club had done was "wrong" and said Southampton were "sorry" to the other clubs involved, "and most of all to the Southampton supporters, whose extraordinary loyalty and support this season deserved better from the club."
Southampton admitted to spying on a training session at Oxford in December and one at Ipswich in April, in addition to the Middlesbrough session.
All three incidents occurred following the appointment of Tonda Eckert as head coach in early December.
Middlesbrough had called for Southampton to be thrown out of the play-offs prior to Tuesday's commission hearing and welcomed the news they had been expelled.
The club said the sanction "sends out a clear message for the future of our game regarding sporting integrity and conduct."
On Wednesday afternoon Boro began selling tickets to their fans for Saturday's playoff final against Hull.
The EFL confirmed that if those two teams did ultimately meet, the match would kick off at 3.30 p.m. BST if Southampton are reinstated on appeal, the match would be played at the originally-scheduled time of 4.30 p.m. BST.