London-based Benoy was employed by developer Parkview to look at the detailed designs - drawn up largely by Grimshaw and Arup Associates - for the retail element of the scheme, and reported the current proposals will fail.
If the scheme does take off, it will involve an overhaul of the site, together with the provision of retail, leisure, tourist and hotel facilities.
But up to a third of the financing for the whole project is dependent on the success of the shopping element.
Benoy assessed whether major retailers could be attracted to take space in the redeveloped power station. The practice claims that its report is the main reason major construction is yet to start on site, despite full planning permission being granted in January.
Carl Francis, associate director at the firm, told the AJ that his work did not provide good news for Parkview.
'Our commercial study showed that the proposals planned for the site simply do not stack up,' Francis said. 'This really isn't good news for the company, as it's the bulk of the project.
'The commercial viability of the scheme is not up to scratch. The developer is planning on getting in a lot of high-end shops. But this hasn't been planned for. There simply are not the facilities these kind of stores need in the proposals that we assessed.
'We understand that Parkview is delaying until the summer while it contemplates what we had to say,' he added.
But Parkview hit back at Benoy's comments, claiming that Francis' remarks were motivated by 'bad blood' because it is no longer working on the project.
'We are already starting some of the enabling work and we expect to be on site by late summer, and we are in negotiations with Bovis Lend Lease about building it,' senior development manager Steve Kennard said.
'We are also working with two other top-class retail architects to make sure that this all works. We are working with the likes of FPDSavills and I can guarantee there is a lot of interest.
'This is clearly about bitterness,' Kennard added. 'There is no doubt we will get this off the ground.'
The Grimshaw and Arup proposals for the site won full planning permission only after deputy prime minister John Prescott decided against calling in the scheme.