Labour leadership: Who are the unions backing?
As well as Labour members, those in trade unions affiliated to the party will be able to vote in the party's leadership race.
The contest is open to members of the 14 unions which back Labour, if they pay a political levy to the party and had registered as affiliated supporters before 12 January.
So which unions back challenger Owen Smith and which support the incumbent Jeremy Corbyn - which have not declared a preference?
Aslef - Jeremy Corbyn
Train drivers' union Aslef has given its backing to Jeremy Corbyn, having backed him in the 2015 leadership contest as well. The union's executive committee endorsed Mr Corbyn in a meeting in July.
General secretary Mick Whelan said: "We are backing Jeremy again because he believes in the core values and the key aspirations of the Labour Party and proposes to run on a platform, at the next general election, which will excite voters put off by the failed focus group-obsessed machinations of the New Labour years."
Bectu - Undeclared
The union for media workers has not taken a position in the leadership race.
"In keeping with past practice Bectu neither made a nomination for a Labour leadership candidate, nor recommended either of those who entered the contest," union officer Tony Lennon wrote in a message to members.
BFAWU (Bakers, Food and Allied Workers Union) - Jeremy Corbyn
At a conference in June, the bakers' union voted that "if there was any challenge to Mr Corbyn's leadership, our union would continue to support the current, democratically elected incumbent and the progressive policies he has put forward".
National president Ian Hodgson wrote that under Jeremy Corbyn "the Labour Party now represented and supported the aspirations of our members".
Community - Owen Smith
Steelworkers' union Community has declared backing for Owen Smith.
The union tweeted that Mr Smith "has shown he is able to unite and lead the Labour Party and more importantly, he can be trusted to lead our country".
CWU - Jeremy Corbyn
The Communication Workers Union confirmed its backing for Jeremy Corbyn at the beginning of August.
"Jeremy is a leader for the millions, not the millionaires," said general secretary Dave Ward.
Fire Brigades Union - Jeremy Corbyn
The FBU re-affiliated to Labour after Jeremy Corbyn's election as Labour leader in 2015, over a decade after splitting with the party in protest at the policies of then-leader Tony Blair.
In July this year the union said Mr Corbyn was "by far the best candidate for firefighters, for the fire and rescue service and for working people in general".
GMB - Owen Smith
The GMB. one of the biggest unions affiliated to Labour, backed Owen Smith to become leader after balloting its membership for their views.
General secretary Tim Roache called on Labour to "unite when all this is over", adding: "GMB members cannot afford for Labour to be talking to itself in a bubble for the next five years while the Tories run riot through our rights at work, our public services and our communities."
Musicians' Union - Owen Smith
The union's executive has made an "advisory recommendation" that members vote for Owen Smith as Labour leader.
John Smith, the union's general secretary, praised his namesake for "offering a second referendum based on approval of the terms of Brexit, which is undoubtedly an attractive option for many musicians concerned about what Brexit might mean".
NUM - Undeclared
The National Union of Mineworkers has not yet declared which candidate it supports.
TSSA - Jeremy Corbyn
The Transport Salaried Staffs' Association, which backed Jeremy Corbyn in 2015, reaffirmed its support for him in July.
"With a new leader at the head of the Conservative Party and the Tories looking set to remain in a prolonged period of bitter rivalry and infighting, it is for Labour to unite and be the source of national unity and stability for the country," the union said in a press release.
UCATT - Jeremy Corbyn
The construction union has nominated Jeremy Corbyn for Labour leader.
"This is an unnecessary and divisive election but now it has been triggered it is essential that Jeremy receives an even larger mandate than he did in 2015 and that the Labour Party then properly unites behind him," said acting general secretary Brian Rye.
Unison - Jeremy Corbyn
The second largest union, Unison, has backed Jeremy Corbyn over Owen Smith.
"Jeremy Corbyn retains the backing of a majority of UNISON's Labour supporting members," said general secretary Dave Prentis. "However, a significant minority backed Owen Smith. Their views will always be respected in our union - that's our proud tradition."
Unite - Jeremy Corbyn
The UK's largest trade union has not changed its position of supporting Mr Corbyn since the last Labour leadership election in 2015, and leader Len McCluskey is one of Mr Corbyn's more outspoken champions.
In a statement in July, the union said a leadership election was "an unhelpful process to be embarking upon at this time".
Usdaw, whose members include retail and distribution workers, has backed Owen Smith, arguing that he stands a greater chance of leading Labour to power.
"Our members cannot afford years of principled opposition, with a Labour Party divided and unable to secure the confidence of voters," said general secretary John Hannett.
The National Union of Rail, Maritime and Transport Workers (RMT) has given its backing to Jeremy Corbyn, but the union is not affiliated to Labour.
"Jeremy has renewed the faith that many rank and file trade unionists have in the Labour Party," said general secretary Mick Cash.