U.S. stocks jumped after strong gains in Europe and Asia. The Dow Jones industrial average and the Standard & Poor's 500-stock index were each up more than 1 percent at the open and managed to maintain those gains through most of the trading day until fading a bit at the finish to close up 0.7 percent and 0.6 percent, respectively.
European stocks advanced, with the pan-European STOXX 600 closing up 3.7 percent and the FTSE 1000 index rising 3 percent. Asian stocks also climbed, with the benchmark Japanese Nikkei ending up 2.3 percent.
Some opinion polls released over the weekend signaled that the campaign seeking Britain's stay in the E.U. had picked up momentum. A vote on the referendum is planned for Thursday.
Last week, the "leave" campaign had gained the upper hand. The poll results over the weekend - showing that 45 percent of voters want to remain in the E.U., compared with 42 percent in favor of a Brexit - boosted the market.
"What the market is telling you today is that, listen, they're probably going to vote to stay, but it's going to be close," said John Canally, chief economic strategist for LPL Financial.
The change in sentiment of the people in British was observed after a three-day hiatus in campaigning. It may be recalled that the campaigning was called off after a member of Parliament who supported staying in the E.U. was killed.
Experts maintain that in the long term, markets would be celebrating a vote to stay in the EU because it would help the UK.