Investors are paying for the privilege of lending their money to the Governments of seven European countries because they are so scared of losing bigger sums elsewhere, says Tower's head of fixed interest.
Speaking at the company's quarterly briefing yesterday, Andrew Lance said that on Tuesday night Belgium had joined other European countries - Switzerland, Germany, Finland, Denmark, the Netherlands and Austria - in offering negative interest on their bonds.
This means investors will receive less money back than they put in when the bonds are paid back.
"It is worse than putting it under a mattress," said Lance.
"People think they will just lose less money [in the sovereign bonds]. It is a phenomenon we have never seen before."