Boris Johnson's office has dimissed plans for more than 400 flats in central Harrow as "deficient".
The Conservative Mayor's deputy said property development firm Dandara's plans to replace the former post office in College Road did not adhere to the capital's planning blueprint.
Dandara's proposals comprise three blocks of flats ranging from three to 19 storeys in height along with shops, a public footbridge over the Metropolitan line, underground parking and a recreation area.
But officials at City Hall published an initial report telling Dandara to resolve the following issues:
Proposed access from William Carey Way
extra contribution towards improved bus routes
extra contribution towards infrastructure
visual impact on views across Harrow
specify where affordable units will be
location of lifts
comprehensive travel plan
maximise affordable housing on site
revisit decision not to use recycled water system
They concluded: "The application does not comply with the London Plan but the possible remedies set out in this report could address these deficiencies."
A Dandara statement said: ''Dandara welcome the GLA comments and are pleased that the mayor is supportive on the significant issues of use, height, impact on the skyline, massing and density, stating 'it is appropriate that it takes on the role of a visual marker'.
"We are confident that the areas in which the scheme does not currently comply with the London Plan, can be addressed before the scheme is presented to the planning committee."