Ballot papers to vote in the Labour leadership contest are being sent out to the 640,000 members and supporters who are eligible to vote.
The vote will decide whether Jeremy Corbyn will remain in power, or if he will be replaced by challenger Owen Smith.
The party estimates that there are 343,500 fully paid up members entitled to vote, as well as 129,000 registered supporters who paid £25 last month, allowing them 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 20,000 applications are still being processed by the trade unions.
Many are expected to cast their votes over the coming days, with a September 21 deadline.
The final result will be announced at a leadership contest on September 24.