Ballot papers are arriving on the doorsteps of more than 610,000 eligible voters in the Labour leadership election with the battle as fierce as any in recent memory.
Here are the answers to key questions about the contest:
:: WHEN WILL THE WINNER BE ANNOUNCED?
The result will be announced on September 12 at a special conference with left-winger Jeremy Corbyn the surprise frontrunner as voting begins.
But frontbench rivals Andy Burnham, Yvette Cooper and Liz Kendall are hoping they can use the three weeks before voting closes on September 10 to turn the tables.
Voters will also choose the party’s deputy leader, with Stella Creasy, Tom Watson, Angela Eagle, Ben Bradshaw and Caroline Flint contesting the number two spot.
London Labour members, registered supporters and affiliates will be able to select the party’s candidate for next year’s mayoral election, with Tessa Jowell, Sadiq Khan, David Lammy, Diane Abbott, Gareth Thomas and Christian Wolmar battling to lead the capital.
:: WHO CAN VOTE?
Many of the 610,753 eligible voters will be casting their ballots this week.
The contest has been plagued by fears of “entryism” from hard left individuals and Tories taking advantage of a rule change which allows members of the public to sign up to vote as “registered supporters” for just £3.
Some 121,295 registered supporters will vote but party officials insist they are working to root out illegitimate voters and will be able to discount their ballots even after they are cast.
They join the 299,755 party members and 189,703 affiliates, including trade union members, who will determine the next Opposition leader.
:: HOW DOES THE VOTING SYSTEM WORK?
The Alternative Vote system is being used with voters asked to rank candidates in order of preference.
If no candidate wins outright with more than 50% of first preferences whoever is in fourth place drops out and the second preferences of their backers are reallocated to the other candidates.
If there is still no winner the third placed candidate is then eliminated with their second preferences similarly reallocated, leaving the remaining two contenders in a head-to-head contest.