LONDON — The world body that investigates cases in which banned chemical weapons are used will begin also assigning blame for those attacks, after a vote on Wednesday by member countries that was bitterly opposed by Russia and Syria, both of which have recently been accused of using the weapons.

The vote grew out of repeated uses of chemical weapons in the civil war in Syria and the charge that a former Russian spy and his daughter were attacked in Britain with a nerve agent.

The investigative group, the Organization for the Prohibition of Chemical Weapons, has sent investigators to determine whether chemical attacks have taken place, violating a treaty joined by nearly every nation, but has not previously pointed fingers at those responsible.