This story was updated at 4:20 p.m. EDT.

U.S. stocks rallied strongly Thursday as the last sweep of opinion polls indicated Britons were likely to vote to remain in the European Union.

Support for remaining in the EU stood at 55 percent, with 45 percent supporting a Brexit, according to an online Populus poll released on Twitter on Thursday while voting was underway on Britain’s referendum. Populus said the survey of 4,700 was carried out Tuesday through midnight Wednesday.

Major U.S. stock indexes closed up more than 1 percent. The Dow Jones Industrial Average added 230 points, or 1.3 percent, at 18,011 while the broader Standard & Poor’s 500 stock index jumped 28 points, or 1.3 percent, to 2,113. The Nasdaq composite, which includes many tech names, rallied 77 points, or 1.6 percent, at 4,910.

The British pound hit a 2016 high and world stocks climbed for a fifth day running.

After a period of stasis, stocks indexes have perked up in recent days as a “remain” vote has looked more likely. Investors are generally fearful of ambiguity.

U.S. markets also took solace in Fed Chair Janet Yellen's two-day testimony this week when she expressed optimism about the economy and downplayed the chances of a recession this year.

“Regardless of the outcome in the U.K., we will see a relief rally in the U.S. today and tomorrow,” said Mohannad Aama, managing director, Beam Capital Management in New York.

A report Thursday showed the number of Americans filing for weekly claims for unemployment benefits dropped to a 43-year low last week, suggesting the employment market remains resilient even though hiring slowed in May. 

In other market-moving news, Macy’s Inc. shares rose 2 percent after Terry Lundgren said he would step down as CEO next year.

Oil prices rose by as much as 2 percent Thursday, shrugging off a smaller than expected draw on U.S. crude stocks as money and equity markets firmed amid optimism Britain will remain in the EU.

Global markets, including commodities, have been on tenterhooks for weeks ahead of Britain’s referendum on EU membership. The majority of results are expected to come in between 0100 and 0300 GMT (9-11 p.m. EDT) following a YouGov poll shortly after voting closes at 2100 GMT (7 p.m. EDT).

Reuters contributed to this report.