The independent disciplinary commission investigating the spygate affair found that Southampton had formed a “contrived and determined plan from the top down” to obtain illicit information for sporting advantage, as they explained their decision to kick the club out of the Championship playoffs.

In a strongly worded assessment of the disciplinary process, the commission also revealed that Southampton had initially denied filming Middlesbrough in training before their playoff semi-final, before ultimately accepting the charges, and that the Saints manager, Tonda Eckert, had “specifically authorised” the spying on three occasions during the season.

In written reasons published on Thursday night, the commission was highly critical of Southampton, described as “the respondent” in the text. “Public confidence is paramount,” they said. “We have concluded that there was, on the part of the respondent, a contrived and determined plan from the top down to gain a competitive advantage in competitions of real significance by deliberate attendance at opposition training grounds for the purpose of obtaining tactical and selection information.”

Middlesbrough initially reported observing an individual filming a training session on 7 May, before their playoff semi-final with Southampton on the ninth. On 8 May, the panel report, Saints responded saying that “the conduct was not part of the SFC culture and that no video footage was captured, transmitted, shared or analysed, when in fact the opposite was the case”.

Southampton accepted the five charges brought against them. In subsequent evidence given to the committee, Eckert also admitted commissioning the surveillance but denied using the material, claiming “he did not find the information useful, that it was wrong or that he did not have regard to it”. Eckert was also forced to admit, however, that the information sought was “sensitive” and that a club “would wish to keep it private in the buildup to a game”.

It is understood that Southampton filmed the training session in part to establish whether key midfielder Hayden Hackney would be fit to play in the tie.

The use of interns to film the training sessions was strongly condemned by the panel. They described “a particularly deplorable approach in its use of junior members of staff to conduct the clandestine observations at the direction of senior personnel” and said the intern who filmed the Middlesbrough session had refused a similar assignment at Ipswich and spoken of the pressure they had found themselves placed under by the club.

Southampton were removed from the playoffs earlier this week, with the final now set to be contested between Middlesbrough and Hull City on Saturday.

Saints were also given a four-point deduction for next season. The club had sought to avert a sporting sanction by pointing the panel towards a previous case involving Leeds, where a spying charge ultimately led to an agreed fine of £200,000.

The panel said the two cases were “sufficiently different” that one should not set a precedent for the other. They went on to argue that a points deduction alone would also not be sufficient. “It would not be effective at this stage in the competition since the financial rewards on offer for a team which is promoted to the Premier League through the playoffs would render any penalty meaningless,” they said.

Earlier, it emerged that Southampton provided footage of their training sessions to the commission to try to prove they gained no material advantage from the saga.

Southampton believe the punishment is out of proportion to the crime and a key part of their plea for mitigation came at the EFL hearing on Tuesday at which Eckert, the chief analyst Nathan Hurst and William Salt, the analyst who was sent to film Middlesbrough, were present.

The club showed in its entirety the footage of Eckert’s session from the Wednesday before the first leg; in other words before Salt’s spying mission. The Southampton players had the day off on the Thursday and on the Friday – the day before the first leg – they did not work on team shape in open play. It was purely a defensive set-piece session, the footage of which they also shared with the panel.

What Southampton and their legal team wanted to show – and which was borne out by the game – was that Eckert’s tactical approach did not change from the session on Wednesday. The club claim their defensive set pieces were not influenced by Salt’s visit to Middlesbrough’s training facilities.

It is part of the reason why Southampton are so upset, albeit they accept they are in the wrong. Eckert, who is battling to hold on to his job amid a Football Association investigation into the conduct of individuals at the club, has argued he did not realise what he did was against the EFL’s statutes. Southampton gave a detailed briefing in pre-season to Eckert’s predecessor, Will Still, explaining the competition’s rules. They did not do the same for Eckert after he replaced Still in early November.

Southampton’s expulsion has prompted Millwall and Wrexham to consider their legal options. The aggrieved clubs are understood to believe they could have grounds to make a claim for compensation.

Millwall and Wrexham could seek to test whether the EFL rulebook has been correctly applied or whether the disciplinary process was flawed, and could argue that because Southampton’s spying on Middlesbrough took place before the playoffs, they should have been replayed without Southampton’s involvement.

Millwall were beaten by Hull in the playoff semi-finals after finishing third and Wrexham missed out after finishing seventh.

There has been speculation in legal circles that one of the parties could seek an injunction at the high court to force the EFL to postpone Saturday’s game but that is regarded as a non-starter owing to the timescale. Any claim would therefore be retrospective for damages.

Hull’s owner, Acun Ilicali, said on Wednesday that his club had received legal advice that they should be automatically promoted to the Premier League as a result of Southampton’s expulsion rather than playing Middlesbrough.

There appears little prospect of Hull pursuing that case in the next 48 hours but they could also pursue a claim for damages if beaten at Wembley in a game worth a minimum of £200m to the winners.

The EFL’s rulebook contains no reference to the process for replacing an expelled team, although its guidance notes state that the playoff final should be contested between two semi-final winners, which will not be the case on Saturday.

Wrexham and Millwall declined to comment.