In responses to employment tribunal claims it is sometimes argued that a tribunal does not have jurisdiction to hear the complaint because the tribunal is ‘estopped’ (prohibited) from deciding an issue because it has already been decided by the Tribunal. This can occur when a claimant has previously made an employment tribunal claim.
The basic position is pretty much commonsense: If an employment tribunal has considered an issue before and made a judgement based on the evidence then that decision is binding and cannot be re-opened except by an application for reconsideration or appeal. Therefore, if I make a claim I was unfairly dismissed and lose that claim it is not open for me to just make a new claim alleging the same thing in the hope that a different judge on a different day might make a different decision and if I were to try to do so then the tribunal would be estopped from considering the issue. There are obviously very good reasons for this rule which ensures final resolutions are made to contested and often emotional cases.
This is a rule that is applied by common law across legal jurisdictions, although interestingly section 137(1) of the Equality Act 2010 restates the rule in legislation: “A finding in relevant proceedings in respect of an act which has become final is to be treated as conclusive in proceedings under this Act.”
One area where issue estoppel can be problematic is on the question of disability. By its very nature many newly received health impairments will at one point of time be unlikely to be a disability (because it is unlikely to be long term) whereas at some point of time the likelihood of longevity changes so that a condition is likely to be a disability. In nearly all disability discrimination complaints a tribunal will need to determine whether the claimant was or was not disabled at the relevant time; if not then the claim will fail.
This raises a potential problem for any worker who has had a previous case dismissed because they were not disabled. Suppose a tribunal decides that a claimant’s depression is not a disability. Now suppose, a year later in a new employment situation the employee believes he being discriminated against in respect of his depression. If he were to make a ET claim and his new employer were to become aware of the earlier judgement then it is quite likely the employer would seek to rely on that earlier decision to argue that the claim should be struck out since the tribunal has already decided he is not disabled and therefore, the Tribunal is unable to reconsider this issue.
Some years ago this problem was considered by the Employment Appeal Tribunal in Mr XX v UKBA  UKEAT/0546/11. The claimant had earlier made a disability discrimination complaint. The tribunal found that the claimant’s dyslexia was not a disability. He later made a subsequently made a subsequent complaint, again alleging disability, although alleging a slightly broader impairment.
The tribunal observed that for an issue to be estopped the issue must be ‘unalterable’ (applying Mills v Cooper  2 QB 459) and in this case aside from the passage of time there was substantially new evidence that post-dated the earlier decision so that the facts from which a judgement on disability could be made was had conceivably altered. Accordingly, the decision that the claimant could not pursue a new disability discrimination complaint based on the same or similar alleged disability in respect of new events that occurred after the earlier decision was overturned. While it is the case that an earlier decision that a claimant was not disabled can still form the basis of a successful striking out application if a claimant can show evidence of ‘alteration’ of the condition then this can often be challenged.