U.S. Federal Reserve Governor Jerome Powell said on Wednesday that the Fed was "ready for whatever happens" in Britain's close-to-call vote on European Union membership on Thursday.

The vote, which echoes the rise of populism elsewhere in Europe and the United States, will shape the future of Europe. A victory for "out" could unleash turmoil on financial markets.

While opinion polls suggested the vote was too close to call, the implied probability of a vote to remain in the EU was at 76 percent, according to Betfair betting odds.

"The markets have been responsive to changes in the polls, clearly, so the vote could certainly bring on significant volatility," Powell told CNBC.

If the leave camp wins, "financial conditions could tighten. It could have an economic effect on the U.K. and the EU," he continued. "We would feel that, as well. We could very well feel that if there were a significant risk-off move."

"Global developments, global weakness ... are really important for the setting of U.S. monetary policy."

Powell told CNBC on Wednesday that Fed policymakers could use a hand from Congress and the White House. "Monetary policy has been really in the ring alone, and could use some help, certainly from fiscal and regulator policy."