Campaigners against Grimshaw's controversial Ellipse building for the Royal College of Art have pledged to fight on after Westminster gave the final go-ahead last week.
Objectors will now be pushing for a call-in by the secretary of state.
James Dunnett, co-chair of DOCOMOMO UK, said he was 'disappointed but not surprised' by the decision.
Grimshaw's £26 million six-storey glass extension to the Royal College would be a 'disaster for the urban and architectural composition created by Cadbury-Brown in 1960 with the original building, ' he said.
Westminster originally approved the project in principle last July (AJ 3.7.03), subject to detailed design refinements to the roof, north elevation and main entrance.
DOCOMOMO, the Twentieth Century Society and the neighbouring Royal Albert Hall have all been fighting the plans.
And Cadbury-Brown, whose Grade II-listed Darwin Building faces partial demolition, has condemned the Ellipse for 'lacking respect' for its context.
However, the RCA's rector Christopher Frayling has consistently rubbished the criticism as 'ill informed rant'.