Jeremy Corbyn's campaign to retain the Labour leadership was boosted this week after the backing of the Communications Workers' Union (CWU). The organisation is a major Labour donor and boasts a membership of more than 190,000 workers, including postal staff, engineers and call centre employees.

Dave Ward, the general secretary of the CWU, said: "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."

The endorsement comes after Unite (1.2 million members), construction union Ucatt (47,000 members) and transport union Aslef (21,000 members) threw their support behind the left-winger.

The Transport Salaried Staffs' Association (20,000) is also expected to back Corbyn. The union shares offices with pro-Corbyn pressure group Momentum and its general secretary Manuel Cortes is a Corbyn backer.

The unions are Labour-affiliates but the Rail, Maritime and Transport Workers union (73,000 members) and the Public and Commercial Services Union (190,000 members) are not officially tied to the party and support Corbyn.

What is less clear is how the unions come to their endorsement decision. The backing of Corbyn from the CWU, for instance, came from its National Executive Council (NEC). A CWU spokeswoman explained the ruling body made the decision because the union was between annual conferences, the last being held in April, and said officials did consult branches.

Ucatt's and Aslef's own executive councils made similar moves, with the former union stressing that the decision to back Corbyn was agreed at its National Delegate Conference in May. Meanwhile, delegates at Unite's policy conference in Brighton "overwhelmingly" backed a motion of support for Corbyn in July.

But some trade union affiliates of Labour are seeking soundings directly from their membership. The GMB, which decided not to endorse a candidate during the party's 2015 leadership election, is currently balloting its 625,000 members, with the consultation closing on 10 August.

"Labour is at a crossroads, so we're keen to have input from as many members as we can as part of the ballot which ends on the tenth of this month," a GMB spokesman told IBTimes UK.

"At the point of the last Labour leadership election GMB chose not to nominate for leader for lack of a clear consensus after consultation with our constitutional committees."

Unison, which has more than 1.2 million members, is conducting a similar process. The union's affiliated political fund committee will decide "in early August" whether to make a supporting nomination in the Labour leadership contest, but the union will canvass views from all of its members opted into its political fund on the issue.

The Fire Bridges' Union (35,000 members) recently re-affiliated with Labour after splitting from the party in 2004 under Tony Blair's premiership. Matt Wrack, the union's general secretary and an ally of Corbyn, is urging the members to back the veteran parliamentarian. So far no trade union has thrown its support behind Owen Smith, Corbyn's sole challenger in the leadership contest.