Jeremy Corbyn's campaign to retain the Labour leadership received a boost after the UK's second largest union endorsed the left-winger. Unison, which represents 1.2 million mostly public sector workers, joins Unite, Aslef, TSSA, Ucatt and the Communications Workers' Union in backing Corbyn.

The move comes after Unison consulted its member over the Labour leadership contest, with a final decision being made by the union's Labour Link Committee.

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Dave Prentis, the general secretary of Unison, said: "Jeremy Corbyn retains the backing of a majority of Unison's Labour supporting members. That's why the committee supported his nomination again.

"It's healthy for people to hold alternative views on the future direction of the party. What's toxic though is for abuse, threats and aggressive language to be considered acceptable – or the norm.

"Labour is in danger of becoming the new 'nasty party' if this behaviour continues unchecked. There's no place in the party for witch hunts against MPs, councillors and party staff."

Corbyn added: "I am proud to have the support of Unison members. Their incredible work, against the backdrop of cuts, privatisation and outsourcing of public services, keeps the services we all rely on running - from the NHS to local government.

"We need a Labour Party that gives them a voice – that halts and reverses the cuts, privatisation, and outsourcing that are ripping Britain's communities and services apart."

The comments follow the GMB union endorsing Owen Smith after 60% of its members backed the former shadow work and pensions secretary in a ballot. The union represents thousands of workers in the UK's defence industry, while Corbyn has been a life-long anti-Trident activist.

Shopworkers' union Usdaw, steel workers' union Community and the Musician's Union have also thrown their support behind Smith.

The news of Unison's endorsement comes as Labour appeals a High Court decision which could see more than 100,000 extra people added to the election's 'selectorate'.

The party's governing body, the National Executive Committee, had decided to block members who joined after 12 January from having a say. A High Court ruled against that decision, in a boost to Corbyn.

Jeremy Corbyn