The bank has previously said that a "significant percent" of its London workforce would be moved if Brexit became a reality.
The bank employs around 5,000 staff at its offices in London.
Earlier, the Lloyds of London insurance market said that after Brexit some of its business would move to the EU.
"We currently employ more than 5,000 people in London, and probably 20% to 30% of our workforce could be affected," the UBS chief executive, Sergio Ermotti, told the Japanese business newspaper Nikkei.
"We believe that London will continue to be an important financial centre, although maybe not as important as it is today," he added.
The UK voted narrowly to leave the EU in the Brexit referendum in June.
Since then the government and many UK businesses have been grappling both with the process of negotiating the divorce - which has not yet been formally triggered - and the eventual economic, social and political consequences of such a move.
Mr Ermotti said that it was too early for the bank to take any decision about moving its operations from London, but it was "well prepared" for any outcome of the impending Brexit negotiations.
The Swiss banking group UBS says that up to 1,500 of its jobs in London may be moved abroad once the UK leaves the European Union.