Fortis Holding, the listed rump of the Dutch-Belgian financial group broken up last year, has countersued a former subsidiary that is now under Dutch government control in a wrangle over securities issued inside the group.

Following a €362.5m ($504m) claim last week from Fortis Bank Nederland, which is owned by the Dutch state, Brussels-based Fortis Holding said on Thursday night it had filed a counter-claim for the same amount.