There are now $11.7 trillion worth of bonds with negative yields

The level of negative-yielding global debt is continuing its climb into the stratosphere.

Following the turmoil of the British vote to leave the European Union and the desire for the safety of government bonds, the amount has jumped to $11.7 trillion. That's a 12.5 percent increase since the end of May, according to a Fitch Ratings report Wednesday.

What's more, the holders of such bonds are willing to hang onto them for even longer, which Fitch said was the biggest factor in the increase. The total of negative-yielding debt with maturities of seven years or longer has swelled to $2.6 trillion, nearly double the amount in April.

"Worries over the global growth outlook, further fueled by Brexit, have continued to support demand for higher-quality sovereign paper in June," Fitch said. "Widespread adoption of unconventional monetary policies, including large-scale bond-buying programs and negative deposit rates, have driven the large increases in negative-yielding debt seen this year."

Thomas Peter | Reuters

People cross a street in a business district in Tokyo, Japan.

With its aggressive easing policies, Japan is by far the global leader in negative yields, at $7.9 trillion, up 18 percent for the month. German 10-year bunds also swung into negative territory — they traded at -0.115 percent Wednesday — as Germany and France now have more than $1 trillion in negative-yielding debt. Switzerland's yields also have turned negative across the board, with the 10-year trading most recently at -0.5274 percent.

Despite the Brexit-related turmoil, U.K. bond yields have stayed positive, with the 10-year at 0.95 percent.

A search for safe havens has driven the willingness of institutional investors buying sovereign debt to accept the negative yields.

"The increasing amount of long-term negative-yielding debt underscores the challenges faced by large bond investors such as insurance companies that need to match long-term liabilities with similar maturity assets," Fitch said. "As more of the global universe of safe assets drops into negative-yielding territory, income for these investors continues to fall."

Fitch said fluctuations in the U.S. dollar also have contributed to the total. The greenback's decline against the Japanese yen accounted for $600 million of the $1.3-trillion growth in dollar-denominated, negative-yielding debt.