Groups such as SAVE Britain's Heritage have told the AJ they will continue to fight KPF's scheme which, it is feared, will radically alter the historic central London market site.
Following the City of London's decision to back the massive commercial development on Tuesday, those opposed to the project are pinning their hopes on a public enquiry.
Adam Wilkinson, of SAVE, says he has already seen a letter from Culture Secretary Tessa Jowell recommending the 40,000m 2retail and office scheme should be called in.
He said: 'I'd be extremely surprised if the development didn't go to a full public enquiry.
'We'd be looking forward to it and to the chance of having our say.'
Although the development, which will result in the demolition of a series of Victorian buildings, was approved by the majority of the City's planning committee, not everyone agreed with the proposals.
Speaking at the meeting one councillor claimed the project was 'an exercise in architectural insignificance' and another that it 'did not enhance the quality of London's built environment'.
The Government Office of London has 21 days from the planning approval to issue an official stop notice on developer Thornfield Property's plans - the first step on the road to a public enquiry.