U.S. Treasury yields rose on Monday with longer-dated yields hitting their highest in more than week, as traders trimmed safe-haven holdings of lower-risk government debt due to polls showing support for Britain to stay in the European Union.
The shift in polls to favoring "Bremain" over "Brexit" comes after last week's killing of parliament member Jo Cox who had been a proponent for the UK to keep its membership in the economic bloc.
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U.S. policymakers including Federal Reserve Chair Janet Yellen have suggested the risk of Brexit on the global economy was a factor behind the central bank's decision to leave interest rates unchanged last week.
"The polls have changed a bit since the assassination of Jo Cox so there is a bit of reassessment on positioning ahead of the referendum," said Larry Milstein, head of U.S. government and agency trading at R.W. Pressprich & Co in New York.
Britons will cast their vote on whether their country should stay in the EU on Thursday.
Benchmark 10-year Treasury yield rose over 4 basis points from late Friday to 1.663 percent after reaching 1.680 percent earlier Monday.
The yield on 30-year bonds was last 2.475 percent, up 4 basis points on the day.
Last week, the 10-year yield slid to its lowest since August 2012 on Brexit fears, the Bank of Japan's decision to refrain from embarking on more stimulus for the moment and Fed officials lowering its outlook on the pace of rate increases.
Top Fed officials scaled back their view on rate hikes following a poor May jobs report and persistent weakness in the manufacturing sector.
Meanwhile, investors awaited Yellen's two-day testimony before Congress, which start on Tuesday, at which she might offer clues on the timing of the central bank's next rate increase.
Interest rates futures implied traders saw a 57 percent chance that the Fed would raise rates by the end of 2016, down from 66 percent a month earlier, according to CME Group's FedWatch program.
While Brexit referendum and Yellen's testimony are the week's main market events, investors will contend with Treasury supply, starting with $26 billion auction of two-year notes at 1 p.m. EDT (1700 GMT).
The Treasury Department will sell $34 billion in five-year notes on Tuesday and $28 billion in seven-year notes on Wednesday. It will also auction $13 billion in two-year floating-rate notes Wednesday.
"We are going to see strong foreign demand for this week's supply," Milstein said. (Editing by Jeffrey Benkoe)