The news will come as a blow to Tate bosses, who have submitted the designs to Southwark council, and are hoping to have the controversial extension built in time for the Olympics in 2012.
To hit the target, a smooth path through the traditionally challenging planning system is essential.
But it would seem that this is ambitious. The source, who is close to the planning team on the project, said it would be a real challenge to meet the timetable being laid out public.
Conservation campaigners are likely to slow the process, with Catherine Croft, the 20th Century Society's consultant director, promising to oppose the development.
'We still think it's a bad idea to build such an extension,' she said. 'And we have written to the council acknowledging our opposition.
'We have great reservations about the impact it will have on the Bankside Power Station, and we think the building should be listed.'
She added: 'They have set themselves a very ambitious timescale, and the committee was quite adamant that it didn't think it was appropriate for that site.'