The yield on Germany's 10-year government bund, Europe's benchmark security, fell below zero for the first time on record, as investors' seemingly insatiable demand for haven assets created another bond-market milestone.

The nation joined Japan and Switzerland in having 10-year bond yields of less than zero. The plunge in yields, which has been driven by European Central Bank's policy of negative interest rates and asset purchases, has accelerated amid a weakening global economic outlook and as polls indicate the “Leave” campaign in Britain's European Union referendum is gaining momentum.

“Nobody buys bunds at these yield levels thinking they are attractive,” said Jussi Hiljanen, head of European macro- and fixed-income strategy at SEB AB in Stockholm. “Demand for haven assets is being driven by fear of Brexit and growth concern. Investors are buying bunds as a hedge against uncertainty.”