TOKYO—The yield on Japan's 20-year government bond is nearing negative territory for the first time, as a scramble for safety after last week's Brexit vote further erodes what positive yields remain in sovereign-bond markets.

The U.K.'s vote to leave the European Union has heightened expectations the Bank of Japan will undertake additional monetary easing, putting further pressure on JGB yields, said Masahiro Ichikawa, a senior strategist at Sumitomo Mitsui Asset Management.

"We expect downward pressure on yields of JGBs with longer maturities," Mr. Masahiro said, with the 20-year yield likely to hit zero by the end of July.

The yield on the 20-year JGB touched a record low 0.04% on Tuesday, while the benchmark 10-year yield hit a record minus 0.225%.

BOJ Gov. Haruhiko Kuroda reiterated Tuesday that the central bank remains ready to take whatever action it deems necessary after the Brexit vote roiled global markets. The BOJ's next policy meeting is scheduled for July 28-29. Mr. Kuroda declined to comment on speculation that the BOJ might call an earlier emergency meeting.

The BOJ is widely expected to take action next month after market turmoil sent the yen, considered a haven asset, to long-term highs. A weaker yen had been a key element of Prime Minister Shinzo Abe's economic program.

The mostly likely options for the BOJ, according to analysts, are expanding its asset-buying program and pushing an interest rate on some bank reserves further into negative territory, from minus 0.1% currently.

Growing concern that global central banks are running out of options to stoke growth has driven a search for assets considered safe and returning positive yields. With many shorter-duration bonds already in negative territory, this has pushed yields on longer-duration bonds sharply lower.

The rise of negative yielding JGBs helped send the total value of global sovereign debt that now offers a negative yield to more than $10 trillion last month, an increase of 5% from April, according to Fitch Ratings.

Yields on benchmark 10-year bonds in Germany, Australia and Switzerland have hit record lows recently, with Germany's 10-year bund yield turning negative this month. In Japan, all government bonds of 15 years and shorter durations already offer negative yields.

The BOJ's massive purchases of JGBs—it now owns about a third of all outstanding JGBs—has helped drive yields lower, as did the central bank's introduction of negative interest rates earlier this year.

If the BOJ undertakes additional easing next month and investors remain risk averse, the yields on 30- and 40-year JGBs may reach zero soon after the 20-year yield turns negative, Mr. Ichikawa said.

Write to Hiroyuki Kachi at hiroyuki.kachi@wsj.com

(END) Dow Jones Newswires

June 28, 2016 02:45 ET (06:45 GMT)