The Premier League is facing the possibility of a legal battle with three of their own clubs after it emerged that Cardiff City, Norwich City and Fulham have joined forces and instructed lawyers to fight the controversial decision not to dock Sunderland points for fielding an ineligible player.

The Observer can reveal that the first steps have been taken by a self-appointed "Gang of Three" to initiate proceedings about the way Sunderland were let off with a six-figure fine despite fielding Ji Dong-won in four league games, spread over seven weeks, earlier this season. Norwich and Cardiff have enlisted Fulham's support to challenge the league, and a legal letter has been sent on behalf of the three clubs arguing that the offence was serious enough to merit a points deduction.

Sunderland's 1-0 win at Manchester United on Saturday helped to relegate Fulham and Cardiff, and leaves the north-east club on a brilliant run of form, three points above third-from-bottom Norwich and on the verge of completing what Gus Poyet has described as a "miracle" escape.

However, there is now the distinct threat of a legal fight continuing into the summer if Sunderland's remarkable sequence of results – also including a 2-1 win at Chelsea and a 2-2 draw at Manchester City – finishes with them securing their top-flight status. The three clubs taking on the league are demanding a review of the Ji case. If that request is turned down, there is the strong chance that whichever club finishes 18th will sue.

League Two side AFC Wimbledon have just been docked three points for fielding an ineligible player and the lawyers for Norwich, Cardiff and Fulham have put together a lengthy list of cases in which other clubs have broken the rules in similar positions and lost points as a result.

Their argument is that Sunderland have been treated as an exception, whereas there is a clear history of evidence – going down the leagues – that this offence ought to merit more than a fine.

Altrincham are the extreme example, docked 18 points in 2006 after signing a player, James Robinson, from Accrington Stanley without realising he had never received international clearance after a previous spell in Iceland. In the vast majority of cases, the punishment has been a three-point deduction.

Demanding an explanation about why the authorities tried to keep the Sunderland case quiet, the legal letter argues that it is unjust that another club might be relegated because of the leniency shown to Poyet's team, and highlights the vast sums of money that would be forfeited by a club dropping into the Championship.

The lawyers also note that Poyet has admitted being surprised the punishment was not heavier, and that Sunderland's manager has stated publicly that Southampton should have been awarded the victory when Ji played in the 1-1 draw at St Mary's in August.

Ji played in three other league fixtures – against Fulham, Crystal Palace and Manchester United. Sunderland lost all of them, which was one of the factors in the league's decision-making, along with the fact that the club had reported their own mistake at the first opportunity. Ji needed international clearance because he had been on loan at Augsburg. Liz Coley, Sunderland's secretary, left her position shortly after the error came to light.

Rule 6.9 in the Football Association's relevant statutes states that "any club found to have played an ineligible player in a match shall have any points gained from that match deducted from its record". It adds that "the board … may also levy penalty points against the club in default".

However, the league believes it has a watertight case because of the wording of the rules. On 4 April, Poyet said: "I'm not saying we should have been docked points, but I would understand if we had. It's incredible. The rules should be clear. It should be one rule. The words 'may' or 'might' in the rules in England, they are unbelievable. They give you a chance to do whatever you like. It shouldn't be may or might. It should be the rule. Yes or no. Then there is no grey area."

Ji also featured in Sunderland's 4-2 win over Milton Keynes Dons in the second round of the Capital One Cup. The case was not heard until Sunderland had reached the semi-finals and the Football League opted against removing them from the competition. Instead, Sunderland received a second fine and agreed to keep the matter on a need-to-know basis, only for the story to be leaked to a national newspaper last month.