Avery Associates has lost out to Jestico + Whiles to design a redevelopment of the Swiss Centre in London's Leicester Square.
The appointment, which emerged this week, comes after British Land sold the site to McAleer & Rushe for £47 million last October.
Plans being considered include redeveloping the site - which sits at the north-west corner of the square and was built in the 1960s to promote Swiss trade and tourism - as a hotel.
Avery Associates' founding partner Brian Avery said: 'We are not really annoyed; just a little disappointed. We don't do a lot of buildings and every one that we do takes a lot of care, so it's sad that we aren't going to build it.
'It was always clear the site could be used as a hotel, so we can't blame the new landowner for not taking on our scheme.' Jestico + Whiles refused to comment.