Nick Terry has stepped down as the chairman of BDP, to be replaced by the boss of the southern region Tony McGuirk.
Terry - who has been chairman of the UK's biggest practice since 2002 - will now take over the reins in the south area, it is understood.
The reasons behind the shock change, the result of elections by the board, are as yet unclear.
It is understood that the move came as a surprise to staffers at the massive practice's London office.
Terry joined BDP in 1972 before leaving two years later to work first in Canada and then for Heery in London. He rejoined BDP as head of one of its three architecture teams in 1990. by Ed Dorrell