Allowing the application for judicial review and quashing the certificate in its entirety
1. Whether a decision was reasonable will depend on the context and circumstances in which it was made:
Lord Dyson, giving the judgment of the court, set out his reasoning as follows:
The Divisional Court was correct in finding that two opposing decisions could both be objectively reasonable. But whether it was reasonable for X to disagree with the reasonable decision of Y would depend on the context and circumstances in which X and Y were acting.
Three authorities were considered as providing a helpful analogy: R v Warwickshire County Council ex p Powergen plc (1998) 96 LGR 617, R v Secretary of State for the Home Department ex p Danaei  INLR 124 and R (Bradley) v Secretary of State for Work and Pensions  QB 114. In each case, the court had emphasised as being of particular importance the fact that the earlier decision had been made by an independent and impartial body, after a thorough consideration of the issues. In these circumstances, the court held that there had to be something more than a mere disagreement on the same material for it to have been reasonable for Y to disagree with X.
In the present case, the Attorney General had disagreed with the decision of the UT (an independent court chaired by a High Court judge) on the very question which the UT had examined in meticulous detail. The Attorney General, however, did not have any additional material and it had not been suggested that the UT had made any error of law or fact.
It was not reasonable, therefore, for an accountable person to issue a section 53(2) certificate merely because he disagreed with the decision of the tribunal. Something more was required. Examples of what would suffice were a material change of circumstances since the first decision or that the first decision was demonstrably flawed in fact or in law.
The fact that a section 53(2) certificate involved making an evaluative judgment (rather than a finding of primary fact) was not material to whether the accountable person had reasonable grounds for forming a different opinion from that of the tribunal.
2. The Attorney General did not have reasonable grounds for issuing the certificate:
The UT had reached its decision after a six day hearing during which it had heard evidence (including expert evidence) and argument at which Mr Evans, the Commissioner and the relevant Departments were all represented by leading counsel. The tribunal had considered all the material and during part of the hearing, even gone into closed session.
The UT’s decision was not appealed by the Departments. Counsel for the Attorney General had not suggested that it contained any errors of fact or law or that its conclusion was unreasonable. The Attorney General simply disagreed with the evaluation made by the UT. He therefore had no good reason for overriding the meticulous decision of the UT reached after six days of hearing and argument. It was therefore unnecessary to consider any further submissions made on behalf of Mr Evans challenging the reasonableness of the views express by the Attorney General (see  of the judgment).
3. Section 53(2) was incompatible with article 6(2) and (3) of the relevant EU Directive, in so far as the information which was the subject of a decision notice was environmental information (see - of the judgment for further discussion). Further, the certificate should be quashed in its entirety because, as regards the non-environmental information, the Attorney General had not explicitly addressed the question of how the competing public interests should be weighed (see  to  of the judgment).