From 1922 to 1981, the Labour Party Leader was elected by members of the Parliamentary Labour Party (PLP) i.e. Labour MPs.
When the party was in opposition there were annual contests, with most elections being unopposed.
In 1981, a special conference was held at Wembley which drew up an electoral college procedure for electing the leader and deputy leader every year.
Forty per cent of the votes were allocated to the Trades Unions, 30% to the PLP and 30% to constituency Labour Parties (CLPs).
In 1993, the rules were changed to require trades unions and CLPs to hold ballots of their individual members (One Member, One Vote - OMOV) during leadership elections. At the same time the electoral college was adjusted to three equal constituent parts.
Tony Blair was the first Labour leader to be elected under this system.