Jeremy Corbyn's campaign to retain the Labour leadership has been boosted after the Communication Workers' Union (CWU) threw its support behind the veteran left-winger. The union, which represents Post Office and Royal Mail workers, has more than 190,000 members.

Dave Ward, CWU general secretary, said: "We are announcing today the CWU's support for Jeremy Corbyn and confirming that we are nominating him for the Labour leadership.

"We need a fundamental change in Labour politics and Jeremy Corbyn recognises this. He is the candidate to drive through the change that ordinary people are crying out for - opposing damaging austerity measures and tackling the housing crisis which is causing misery for so many.

"It's policies like Jeremy's announcement on extending the recognition for trade unions which we believe will make a real difference to the working lives of millions. Jeremy is a leader for the millions, not the millionaires, and the CWU is proud to support him."

The endorsement comes after transport union Aslef and construction union Ucatt backed Corbyn, who won almost 60% of the vote in Labour's 2015 leadership contest. Unison, a major Labour donor, has launched a consultation of its 1.2 million members to help decide who to back in the leadership contest.

The Transport Salaried Staffs' Association (TSSA) is also expected to back Corbyn. The union's general secretary Manuel Cortes is a supporter of the Islington North MP and pro-Corbyn pressure group Momentum works out of the TSSA's offices.

Corbyn's sole challenger is Owen Smith, the former shadow work and pensions secretary. The Pontypridd MP has promised to reintroduce wage councils, scrapped under Margaret Thatcher, and repeal the controversial Trades Union Act.

Corbyn has also promised to axe the legislation, which requires a 40% strike ballot threshold to be met for crucial public service. In addition, the Labour leader said he would introduce French-style collective bargaining laws if he gained power.

"The best way to guarantee fair pay is through strengthening unions' ability to bargain collectively – giving employees the right to organise through a union and negotiate their pay, terms and conditions at work," Corbyn wrote in The Observer on Sunday (31 July).

"That's why it should be mandatory for all large employers, with over 250 staff, to bargain collectively with recognised trade unions."

The key dates for the Labour leadership

  • 18 July: Signing up as a registered supporter to vote in the election begins.
  • 19 - 20 July: Number of MP nominations for each candidate to be published.
  • 20 July: Deadline for people to sign up as a registered supporter closes.
  • 22 August: Ballot papers will begin to be sent out around the UK, but to Labour Party members only.
  • 21 September: Deadline for ballot papers to be returned is midday.
  • 24 September: The election result will be announced at a Leadership Conference in Liverpool.