Campaigners who object to the £775 million Paddington Cube development have won permission to fight a fresh battle over the controversial scheme.
The decision, which was made in the High Court despite the project being given the official go-ahead, is likely to dismay the developers.
A senior judge ruled that Save Britain’s Heritage has arguable grounds for seeking a judicial review, after communities secretary refused to call in the project for a public inquiry.
The group is accusing the minister of unlawfully failing to give reasons why he decided not to intervene over the construction of the “floating” office block near Paddington station.
Health chiefs have expressed "serious concerns" that the development would impede ambulances at St Mary's Hospital, which is one of the capital's four major trauma centres and treated victims of the Grenfell Tower disaster.
It is also being opposed on the grounds that it will have a negative impact on an important conservation area in the heart of the capital.
Imperial College Healthcare NHS Trust, which runs St Mary's, joined Save Britain's Heritage and the Victorian Society earlier this year in asking the secretary of state to call in the project.
The judge heard that Westminster City Council, which initially gave its approval last December, has now issued a final permission for the 14-storey block.
Saira Sheikh QC, appearing for the planning authority, argued that, as permission had been given, there would be no purpose in the legal challenge proceeding.
Richard Harwood QC, appearing for Save Britain's Heritage, disagreed and said the Communities Secretary was under a legal duty to give reasons.
If the legal challenge succeeded and the decision not to call in was quashed, the Communities Secretary would have to reconsider the case, Mr Harwood told judge Sir Ross Cranston.