Northern Rock PLC, the nationalized U.K. lender, said Thursday that its massive Granite securitization vehicle had hit a key trigger that would cause it to unwind, meaning some bondholders may have to wait many years longer than they expected to be repaid.

Granite, a so-called master trust that was set up to fund mortgage lending by issuing mortgage-backed securities, hit the trigger "due to the current seller share being less than the minimum seller share on two successive distribution dates," Northern Rock said.

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