A coalition of hospitals - St Mary's, the Royal Brompton and Harefield - worked with the practice on the ill-fated project, which would have seen a massive PFI scheme built on a site in the Paddington basin.
SOM won outline permission for their masterplan in August 2002 after charging £1,040,000 for its services, the report said.
However by November in the same year it had already become apparent that the vast scheme had outgrown the limitations of the practice's project.
The project was then handed to Farrells, who drew up a fresh masterplan for the new hospital.
By 2005, it was reported that the three trusts had spent £7.35 million on consultancy fees and still had failed to win planning permission for the project scheduled to replace the SOM design.
The full NAO report is available at http://www.nao.org.uk