Staff were informed of his departure in a memorandum from Hans-Peter Bauer, the global head of fixed income and derivatives.
Sources said Mr Briance, 43, who joined UBS from Credit Suisse First Boston in 1991, had left after management differences. They said his management style was very different from that of Mr Bauer, who arrived from Switzerland at the start of 1996 when UBS restructured to refocus the running of its global operations out of London instead of Zurich.
Mr Briance's role in the restructuring was to head fixed income globally and most importantly to increase the bank's presence in the US, where it had it struggled to make a impact.
Sources said he had been successful in his role, hiring some high-profile staff in the US in the last year. The sources did not believe Mr Briance had another job lined up immediately but thought his reputation in the markets would make him an attractive hire for one of UBS's competitors.