The Competition Appeal Tribunal’s (CAT) recent judgment in Spreadex Ltd v Competition and Markets Authority [2026] CAT 24 – a challenge to a CMA merger decision – provides a useful reminder that there is a high bar to succeeding with an “irrationality” challenge. Running such a case will not provide a licence to re-run the merits of a decision.

Evidence being open to interpretation is not sufficient to show a decision is irrational[1]

Irrationality is one of the classic grounds for a judicial review challenge to a decision by a public body. It refers to a decision so unreasonable that no reasonable authority could ever have come to it, and sets an extremely high threshold; catching only decisions that are outrageously defiant of logic, morally indefensible or simply inexplicable.[2] As we highlighted in our  top ten takeaways on judicial review cases, it has a much lower success rate than pleading illegality, primarily because of the Administrative Court’s deference to authorities as experts in their field. The approach in this case reinforces that even specialist tribunals like the CAT will take a similar approach. 

Here, Spreadex, an online betting operator, sought to challenge the CMA’s decision that its acquisition of Sporting Index’s B2C betting business would lead to a substantial lessening of competition in the UK market for spread betting services, and therefore required divestment. In particular, Spreadex claimed that, given evidential limitations, the CMA’s analysis of the “counterfactual” to determine whether the merger would be anticompetitive was unreasonable. The relevant evidence concerned the viability of alternative future outcomes, such as winding down the business or accepting an alternative bid, so was innately difficult to obtain. Key inputs were uncertain, for example, the indicative value of rival bids based on incomplete due diligence and provisional cost modelling.

Ultimately, the CAT agreed with the CMA’s argument that this was an attempt to selectively re-argue the merits of a series of evidential points”.[3] It acknowledged the CMA’s discretion to choose between equally reasonable inferences which can be drawn from the same evidence, particularly because selecting the correct counterfactual is inherently hypothetical and uncertain. The CAT made clear that the CMA’s power to decide is not subject to a merits review,[4] it must refrain from “second guessing” the educated predictions made by an expert and experienced decision-maker.[5]

Equally “reasonable” public law decision-makers can come to different but legally rational conclusions.[6]

This case reinforces the existing judicial review standard of irrationality. For potential appellants, the implication is not to shout “irrationality” louder, but to focus on genuine public‑law error. Creativity here means precision: identifying concrete flaws in the decision‑making process rather than relitigating the merits.