Labour is sending out 3.4 million ballot papers to party and trade union members, as the contest for the deputy leadership reaches its final stages.
Labour's new deputy leader will be announced on 24 June
The six contenders are travelling around the UK seeking support, with the result to be announced on 24 June.
The winner replaces John Prescott as deputy Labour leader when Gordon Brown becomes prime minister.
Hilary Benn, Hazel Blears, Jon Cruddas, Peter Hain, Harriet Harman and Alan Johnson are the candidates.
They all received enough nominations from Labour MPs to progress to the nationwide ballot.
This is in contrast to the the Labour leadership, where Mr Brown's only rival - left-winger John McDonnell - did not get enough backing to go through.
The eventual winner is not guaranteed to follow Mr Prescott as deputy prime minister, as it will be Mr Brown's choice - as prime minister - whether he even wants to appoint someone to this role.
The deputy leader is chosen by an electoral College made up of one third MPs and MEPs, one third party members party members and one third trade unionists and socialist societies.
People voting will rank the candidates, so unless someone gets 50% of the vote in the first round, the least backed candidate will drop out and their backers' second preferences reallocated to the remaining candidates.
The process will continue until one candidate gets over 50% of the vote.
The election system make it difficult to predict at this stage who will win.