Britain's vote in June on whether to leave the European Union is shaping up to be the next global event to impact the US Federal Reserve's decision-making on when to next raise interest rates. 

San Francisco Fed President John Williams has become the latest US central banker to suggest that the vote on Brexit could play into deliberations. 

The Brexit referendum in the UK comes a week after the Fed's next policy meeting.

"Clearly if there's an expectation that it actually will pass and the markets will react to that then we have to take that into consideration in terms of how it affects the US economy and the outlook," Williams told Reuters on the sidelines of a conference at Stanford University. 

Williams said that given strong US jobs gains and signs of stirring inflation, two or three rate hikes this year seem reasonable. 

His views on Brexit echo those of Atlanta Fed President Dennis Lockhart, who is attending the same conference, and who said that Britain's vote "is a big deal" that deserved close attention.

"I think it's possible that as the markets absorb information running up to the Brexit vote, that the markets are volatile," he said. 

Polls show a tight race in Britain with the outcome likely uncertain before the Fed meets next on June 14-15. 

The Fed raised rates for the first time in a decade in December from near zero but have since stood steady, partly due to the global economic uncertainty. 

Dallas Fed President Robert Kaplan said during a visit to London last week the Brexit vote will be a factor in the Fed's June decision unless the outcome of the vote is very clear ahead of time. 

However St Louis Fed President James Bullard said earlier that he does not see Britain's referendum as a global stress event.