HBG Properties withdrew on the eve of signing contracts with Birmingham city council because its parent company, Royal BAM Group, overruled the decision to invest.
The decision has left Birmingham council scrambling to find a backer to develop the Great Charles Street site in Birmingham's city centre, and means that Gensler has missed out on a highly lucrative contract.
HBG offered to pay £23 million for the site, making it the most expensive piece of land sold by the local authority.
The company announced plans to build a mixed-use scheme including offices, housing, car parking and a new footbridge linking the Jewellery Quarter with the city centre.
However, just before signing, HBG mysteriously decided to pull the plug on the scheme.
A spokesman for HBG Properties, said: 'The company regrets any embarrassment caused by this decision, but emphasises that its withdrawal from this particular scheme was covered by certain conditions contained in the original bid.
'These conditions are confidential,' he said. 'This situation does not in any way limit HBG Properties' continued commitment to, and investment in, the Midlands generally or Birmingham specifically.'
Birmingham council has played down the effects of the withdrawal, and remains resolute that the scheme will find backing from one of many potential developers.
Ken Hardeman, the Birmingham cabinet member for regeneration, said: 'We are obviously disappointed that HGB didn't deliver what they offered. But we have had so many interested parties, I am confident we will secure a quality scheme.'