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The June 20-24 poll showed that 46.6 percent of likely American voters supported Clinton while 33.3 percent supported Trump. Another 20.1 percent said they would support neither candidate.
Trump had enjoyed a brief boost in support following the June 12 mass shooting in Orlando, Florida, as he doubled down on his pledge to ban Muslims from entering the country, cutting Clinton's lead to nine points.
But Trump's rise in popularity appeared to be only temporary, unlike his lasting surge among the Republican field last year after the attacks in Paris and in San Bernardino, California.
Clinton's 13.3 percentage point lead is about the same as she had before the Orlando attack.
Trump's slip this week came as he struggled to show that he can keep up with a Clinton campaign apparatus that has dwarfed his in size and funding.
Campaign finance disclosures released earlier this week showed Trump started June with a war chest of just $1.3 million, a fraction of Clinton's $42 million. Trump sought to ease concerns among his allies by saying that he could tap his "unlimited" personal wealth if needed, and also by bolstering efforts to raise money through fundraising events and online donations.
Meanwhile, Republican leaders including House of Representatives Speaker Paul Ryan and Wisconsin Governor Scott Walker continued to express reservations about their new standard bearer, who has angered some in the party with his fiery rhetoric.
Ryan and Walker both said over the past week that they felt Republicans should follow their "conscience" when deciding to support the party's likely nominee, instead of urging party members to support him.
The poll only captured some of the voter reaction to Britain's decision in Thursday's referendum to exit the European Union, a move that some pundits say suggests Trump's insurgent candidacy has tapped into a broad and powerful anti-globalization wave sweeping Western countries.
The Reuters/Ipsos poll was conducted online and included interviews with 1,201 likely voters in all 50 states. It has a credibility interval, a measure of accuracy, of 3.3 percentage points.