The government’s role in a disputed planning application to demolish a historical Royal Mail sorting office next to Paddington station in London and replace it with a 19-storey glass tower is to be scrutinised by judges.

The case, to be heard at the Court of Appeal on 19 July, will examine why official advice questioning the merits of the project, shared with Sajid Javid when he was the local government secretary, was not disclosed, in breach of the government’s published policy.

The tower – Paddington Cube – was designed by Renzo Piano, the Italian architect, for the Sellar Property Group, the company behind the Shard, Britain’s tallest building.

The project faced legal challenges from St Mary’s Hospital because it would lengthen ambulance journey times, and from heritage campaigners because the 111-year-old post office building was in a conservation area. But in March last year, Javid, who is now the home secretary, declined to call in the scheme, and gave no explanation. This paved the way for a major transformation of the area around Paddington station.

“No one is saying you can’t have new development in a conservation area but it’s meant to respect the character and the sense of place,” said Henrietta Billings, director of Save Britain’s Heritage, which is bringing the legal challenge. “If you can build a glass tower block like this at this scale, completely out of character with its neighbours, it means something in the system isn’t working.”

Westminster council, which approved the planning application, has acknowledged that the Edwardian sorting office is a “building of merit”. It also accepted that the Cube would have a “major impact on the character and appearance of the Bayswater conservation area” and would “harm the conservation area and the setting of listed buildings including the Grade I-listed Paddington train station”.

Facebook Twitter Pinterest Westminster council acknowledged that the former Royal Mail sorting office was a ‘building of merit’. Photograph: Buildington

The court challenge will focus on government policy, dating from 2001, stating that ministers must publish the advice given to them by civil servants if they choose not to call in a planning application.

The advice on the Paddington Cube has not been made public. Redacted correspondence from the civil servants advising the government suggests that if it had been “there would likely be more challenges to [the] decision and greater pressure exerted by the media and public”.

However, a witness statement from a civil servant working in the department’s planning casework unit explained that there was no need to publish the advice: “From early 2014 the practice of not giving reasons in non-intervention decision letters was implemented. This change of practice was promulgated internally via team leaders but was not communicated externally.”

Billings said she could not explain why the policy change was not made public. “In 2014 the process changed but this wasn’t announced in parliament. It’s slightly mysterious as to how it changed. There was no official announcement and that’s part of the reason for our challenge: why was a policy that was announced to the House of Commons suddenly changed without any formal announcement?”

Answering this question could decide the fate of future planning applications, Billings suggested.

“If you don’t know the reason why a decision was made, it’s not possible to know if it was a lawful decision and there was no mechanism to challenge it. In the interests of transparency and open government it’s important to know how these decisions were made.”

The planned demolition of the building, which was sold off by Royal Mail and is currently used by Cross Rail, has drawn parallels with the move to knock down St Pancras station, saved only by a campaign led by the poet Sir John Betjeman.

“It harks back to the mass clearances of the 1960s,” Billings said. “But these buildings are beautiful, they have lots of decorative detail and are built to last. No one would dream of knocking St Pancras down now and it’s really important to understand that fashions in architecture do change over time and you have to be really careful not to completely obliterate these buildings in the quest for new futures. Once they’ve gone, that’s it, you can’t get them back.”

A spokeswoman for the ministry of housing, communities and local government said it would not be commenting because of the legal action.