His group, the RPDU, condemned the decision not to register it as "unwarranted" and "erroneous".

BBC: Mr Kasyanov is one of Vladimir Putin's rare public critics

She denied the decision was political, pointing out that the RPDU could reapply as many times as they wished.

BBC: Mr Kasyanov is one of Vladimir Putin's rare public critics

The RPDU said the decision was not unexpected.

BBC: Mr Kasyanov is one of Vladimir Putin's rare public critics