Concerns have been raised over the amount of demolition required to make way for two proposed east-west tunnels in London.
Mayor Boris Johnson last week set out plans for underground roads from the A40 at Park Royal to the A12 at Hackney Wick, and from the A4 in Chiswick to the A13 in Beckton.
Johnson asked Transport for London to further develop these proposals after the body found they could cut congestion in the heart of the capital by a fifth.
But architecture campaigners the Victorian Society expressed “serious concerns” over the plans.
Director Christopher Costelloe said: “We must be careful not to repeat the mistakes of the mid-20th century, which saw vast areas demolished for traffic schemes which are often regretted today.
“The likely amount of demolition necessary must be weighed against the fact that the suggested benefits of the scheme, such as a 20% reduction in central London congestion, are by no means certain.”