In November last year, a High Court judge upheld a decision by the then communities secretary Sajid Javid not to call in plans for the office-led redevelopment of a former Royal Mail site next to Paddington station in central London.
The proposed "Paddington Cube" is a 19-storey, 54-metre-tall office tower next to the grade I listed station and within the Bayswater conservation area.
Campaign group Save Britain's Heritage (SAVE) claimed that the glass tower would permanently harm heritage assets in the conservation area.
The High Court judge ruled that Javid had not been obliged to give his reasons for allowing Westminster City Council take the final planning decision on the scheme.
But today, the Court of Appeal ruled that Javid should have disclosed his reasons for refusing to call in the planning application.
Although work on the Paddington Cube will still go ahead, SAVE said the court's decision has crucial implications for transparency and ministerial accountability.
Appealing against the High Court ruling, Richard Harwood QC, for SAVE, accepted that there was no statutory duty on Javid to explain his reasoning, but argued that, since 2001, it had been established government policy to give reasons for refusing to call in planning applications.
The policy was announced in a green paper laid before Parliament in December 2001.
However, the practice of the Department of Communities and Local Government - now the Ministry of Housing, Communities and Local Government - changed in January 2014 and, since then, such decisions have routinely been given without reasons.
Harwood said the shift in practice came about because civil servants were simply "unaware" of the green paper.
Upholding SAVE's appeal today, Lord Justice Coulson said there could be no doubt that, in 2001, the government had made "an express promise that reasons would be given" for decisions to not call in planning applications.
Ruling that SAVE had a "legitimate expectation" that reasons would be given in the Paddington Cube case, he said: "I do not consider that the relevant promise has ever been withdrawn."
He added: "It is a recipe for administrative chaos if a legitimate expectation can be generated by an unequivocal ministerial promise, only for it then to be lost as a result of an unadvertised change of practice."
Matters got "worse" for the department, when it emerged that "nobody in the department recalled or had in mind" the policy promise made in 2001 when the practice of giving reasons for call-in decisions was changed in 2014, said the judge.
"Mr Harwood was right to submit that the change in practice relied on by the secretary of state was brought about by ignorance of the 2001 policy promise," he added.
Government lawyers argued that the change in practice was now well established and reflected on departmental stationery, but the judge ruled: "I do not accept the proposition that a policy which has been promised can be withdraw simply by a change in the template of letters."
The judge, sitting with Sir Andrew McFarlane and Lord Justice Singh, concluded that SAVE's legitimate expectation that reasons would be given "did not come to an end as a result of the confusion and muddle" within the department when the practice was changed in 2014.
The court formally declared that SAVE should have been given reasons for Javid's refusal to call in the application.
Marcus Binney, executive president of SAVE, said: "In recent years it has become increasingly hard to secure public inquiries into even the most controversial schemes which have attracted major opposition both locally and nationally, and which are often approved by councils in complete disregard of their own planning policies – as in this case.
"The ringing judgment, calling ministers and civil servants to account, and criticising a major Whitehall blunder, will resonate through the planning system."
Henrietta Billings, director of SAVE Britain’s Heritage, said: "This is a fantastic result that opens up the decision making process for highly contested major schemes across the country. It literally changes the landscape of decision making – and is a major victory for openness and transparency."
R on the Application of Save Britain's Heritage v Secretary of State for Communities and Local Government. Case Number: C1/2017/3462