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."