Nationwide is suing Birmingham solicitors Eversheds for upwards of #20 million for alleged negligence in a property fraud.
The building society has already settled what was originally a #26 million claim against Dunlop Haywards, part of the property services group Erinaceous, which it alleged has perpetrated a property valuation fraud.
This is believed to have involved a commercial building in Wales on which Nationwide offered a #14.25 million loan after being told it was worth #15 million with vacant possession or #19 million fully let.
In the event, Nationwide says the value turned out to be no more than #850,000. A spokesman for the society stressed "We are not alleging the solicitors acted fraudulently in any way. It is purely negligence."
Eversheds stated: "In the ruling on the Nationwide vs Dunlop Haywards case, Mr Justice Simon quite clearly determined that an employee of Dunlop Haywards acted deceitfully.
"Like Nationwide, Eversheds who acted for them, was subject to the fraud perpetrated by others. To protect their position, Nationwide has issued proceedings against Eversheds."
The shares fell 5.5p to 53p.