The Public and Commercial Services (PCS) union has agreed that it will back Jeremy Corbyn against other contenders for Labour leader.
The union's ruling body also agreed to step up anti-racism campaigning at an emergency meeting to discuss the fall-out from Brexit and how it impacts on its members' jobs.
At the meeting, members noted that Mr Corbyn had promised to restore national pay negotiations in the civil service and oppose further cuts to pay and redundancy terms, and to repeal the Trade Union Act.
PCS general secretary Mark Serwotka said: "Our union supports Jeremy Corbyn against what is clearly a choreographed and undemocratic attempt to unseat him.
"Using the EU referendum result as cover is inexcusable when the urgent need is for Labour and the unions to unite to combat the rise of racism."
The leader of the UK's biggest trade union has said the country is 'riven' as a result of the EU referendum.
Len McCluskey told ITV News that the trade unions can help broker a peace deal and reunite the party now the "coup has failed".
Labour leader Jeremy Corbyn said he would 'reach out' to MPs that oppose his leadership but said they should 'respect democracy'.