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.

Notices of intended prosecution claimed 991 drivers had broken a 40mph (64km/h) speed limit in Conwy tunnel last October.
But the limit had been imposed for night maintenance only and not lifted in the morning as it should have been.
Within days the drivers got an apology in the post.
North Wales Police released the figures in a Freedom of Information reply.
The force said: "The issue was caused by an administration error surrounding the enforcement period.
"North Wales police do not record the cost of cancelling notices."

The suspended Uefa president, 60, was banned by the world governing body's ethics committee in December, along with its president Sepp Blatter.
Both were found guilty of breaches surrounding a £1.3m ($2m) "disloyal payment" to Platini.
Blatter's appeal against his own ban is due to be heard on Tuesday.
The former France international and the Swiss have both denied any wrongdoing.
Platini was also fined £54,000 ($80,000) after being found guilty of breaching Fifa's code of ethics.
Both Platini and Blatter said the payment was made following a verbal agreement between the pair when the Frenchman worked for Blatter from 1998 to 2002, though it was not paid until nine years later.
The Fifa appeals committee, which is chaired by president of the Bermuda Football Association Larry Mussenden, has the power to reduce, increase or overturn the ban.
The ethics committee's investigatory chamber has confirmed it is appealing against the sanctions imposed on Platini and Blatter on the grounds that they are too lenient.
Uefa said last month that it will not hold an election for its presidency until Platini's appeals process has been concluded.
Platini had at one stage been favourite to succeed Blatter as Fifa president, but the opening of ethics proceedings against him and subsequent ban ended any chance of that happening.
Fifa's presidential election is due to take place on 26 February.
On Friday, the committee banned the governing body's former secretary general Jerome Valcke for 12 years for multiple breaches of the code of ethics.
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.
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Nearly 1,000 drivers were wrongly sent speeding notices after a temporary limit on a north Wales road was not lifted, figures have shown.
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Michel Platini's appeal against his eight-year ban from all football-related activity will be heard by Fifa's appeals committee on Monday.