More than 640,000 Labour Party members and supporters will be eligible to vote in the leadership contest between Jeremy Corbyn and Owen Smith, the party has said.
As the first ballot papers were being sent out, the party estimated there were 343,500 fully paid up members entitled to a vote in the election.
Their numbers will be supplemented by 129,000 registered supporters who made a one-off payment of £25 last month so they would be able to vote.
A further 168,000 affiliated supporters who are members of affiliated organisations such as trade unions also have a vote - although that number is expected to rise as the unions are still processing some 20,000 applications.
More than 50,000 applications to become registered supporters were rejected for "technical reasons" - such as duplicate applications, where people hit the refresh button on the online form multiple times, or failure to make the payment deadline.
Party members will receive their ballots by post as well as an email with an online ballot code - giving them a choice as to how they vote - but supporters will receive only an online ballot.
Online voting codes will be sent out during the course of Monday, Tuesday and Wednesday although Labour said the numbers involved meant that it would be "a number days" before all the ballot papers had been despatched.
Voting closes at midday on September 21 and the final result will be announced at a special leadership conference in Liverpool on September 24.