Allies of Barack Obama have warned of chaos if unelected "super-delegates" overturn the will of grassroots Democrats when the party chooses its presidential candidate at its national convention in August.

The Illinois senator is leading in the primaries battle for elected delegates but the contest could yet be tipped in Hillary Clinton's favour by nearly 800 party officials - known as super-delegates - who comprise a fifth of the overall 4,049 delegates voting at the convention.