Spanish lender Banco Santander has said it will not redeem a risky kind of bank debt, a move analysts said could ripple across a largely untested corner of the bond market.

The bank’s decision not to “call” the €1.5bn ($1.33bn) bond is believed to be the first of its kind in the so-called additional tier 1 bond market. AT1 debt is considered particularly risky because it has a perpetual maturity — leaving issuers free not to ever repay bondholders.