London mayor Ken Livingstone has threatened to refuse permission for plans by Penoyre and Prasad to develop a laboratory facility next to Charles Darwin's home Downe House near Biggin Hill.
The Government Office for London has also issued an Article 14 holding directive after Bromley council announced it was set to give the scheme the go ahead.
Livingstone is concerned the designs for a low-rise conference and science facility will negatively impact on Downe House and the surrounding Green Belt.
He has ordered that changes be made before the project can be reassessed.
A letter from the mayor's planning manager, Giles Dolphin, to Bromley council states:
'This may mean that a reduction of the proposed bulk design of the building is needed, and the footprint of the building is not increased in the future.
'The mayor recognises that there are clear benefits for establishing this centre, ' it goes on.
'However the impact of the proposed Charles Darwin Building needs to be reduced.'
And a Government Office for London source added that planning inspectors are 'genuinely concerned about its size, its impact on the listed Downe House and the effect on the Green Belt'.
Penoyre and Prasad refused to comment on the move until the Government Office for London has decided whether the scheme should go to a planning inquiry.