Hawkins\Brown Architects has beaten Foster and Partners for the chance to design the hugely significant redevelopment of Parliament Square, London.
The practice has been appointed lead consultant to take forward a masterplan originally drawn up by a design team including Foster and DSDHA.
A Foster-led team then lost out to Hawkins\Brown in a competitive interview process.
The decision will come as a massive disappointment to Norman Foster, who was vying for the chance to develop his own long-term vision for the square, set out in his World Squares Masterplan.
Foster's vision was part of an overall development running from Trafalgar Square, south through Whitehall and ending at Parliament Square.
Foster and Partners completed the revamp of Trafalgar Square in 2003, and fully expected to continue its work in Parliament Square.
In fact, it was following the success of Foster's Trafalgar Square that London Mayor Ken Livingstone decided to forge ahead with the redevelopment of Parliament Square, instead of Whitehall.
The decision will prove difficult to swallow for Foster, particularly in its echoes of the fitting out of the Swiss Re Building - a project for which Foster was overlooked in favour of bennett interior design.
The Parliament Square scheme is part of Livingstone's edgling 100 Public Spaces initiative, in which Hawkins\ Brown has already enjoyed relative success with its Gillett Square in Hackney.
The practice will work closely with Deborah Saunt of DSDHA. Saunt will lead a team including landscaping expert EDAW, conservation guru Montagu Evans and lighting designer Jason Bruges to maintain continuity of the masterplan.
Practice principal Roger Hawkins said: 'We're delighted to have won the scheme; it's a fantastic achievement.
'Parliament Square is one of the most important spaces in the country and even the world.
People come here from all over the world, and the Houses of Parliament still have a sway over Commonwealth countries around the world.
'This project needs to work on a rainy day in April with only two people in the square, as well as a coronation with up to 100,000 people, ' he added.