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."
Home Office staff will go on strike today in the latest phase of three months of industrial action by civil servants in a dispute over pay, jobs and terms and conditions.
There will be walkouts for the rest of the week in different sections of the department, the Public and Commercial Services union said.
Immigration and asylum staff will strike today, with Border Force staff taking action at a later date.
Mark Serwotka, the general secretary of the Public and Commercial Services (PCS) said, today's strikes by Revenue and Customs staff are part of an "ongoing campaign" to "cause disruption for the Government at key times". He added:
Thousands of Revenue and Customs workers will stage a half-day walkout today, marking the latest phase of industrial action by civil servants.
Members of the Public and Commercial Services (PCS) union, which has over 50,000 members in HM Revenue and Customs, will stage picket lines outside offices across the country.
A 24-hour strike in the Home Office and UK Border Agency planned for today was postponed after a legal challenge.
It will now be escalated to a week-long series of walkouts across various parts of the department in a fortnight's time.
Thousands of staff from Revenue and Customs will go on strike tomorrow in the latest phase of months of industrial action by civil servants in a row over jobs, pensions and terms and conditions.
Members of the PCS Union will stage a half-day walkout until lunchtime. Officials say it will disrupt the start of the new tax year.
Asked if he was talking with other union leaders about a general strike, PCS general secretary Mark Serwotka told Murnaghan on Sky News:
A union leader has confirmed he is in talks with counterparts about staging a general strike.
Mark Serwotka, general secretary of the Public and Commercial Services Union (PCS), said discussions were under way about action that would show people can "fight back".
It comes after it emerged last week that Unite had submitted documents to the TUC calling for a 24-hour general strike against austerity measures.
A walkout by Public and Commercial Services (PCS) union members is set to hit workplaces across the UK this afternoon.
Starting at 1pm, PCS members will take part in a half-day strike to tie in with a three month campaign over pay and pension disputes.
PCS general secretary Mark Serwotka said: "These walkouts, following our strike on budget day, are part of an ongoing campaign of industrial action and protests to put pressure on a government that is refusing to even talk to us.
"We have asked for talks but ministers and senior officials have refused, so we are taking action to oppose and shine a light on what are deeply unfair and unnecessary cuts to the living standards of hard-working public servants."
Civil servants will stage a 24-hour strike on Budget day today, in a row over pay, pensions and working conditions.
It is thought up to 250,000 members of the Public and Commercial Services union (PCS) will join the walkout, which will hit job centres, border control at airports, tax offices and courts.
A half day of action has also been called on April 5th, as part of a three month long campaign of action.
Picket lines will be placed outside government offices, museums, galleries and the Houses of Parliament.
Civil servants have planned a second walkout in a row over pay, following their Budget day strike.
Almost 250,000 members who work in government departments and its related bodies will hold a half-day walkout on April 5, the Public and Commercial Services union said.
A 24-hour stoppage has been called by the union on Wednesday after accusing the Government of refusing to negotiate over cuts.
A three-month programme of industrial action is also being planned, along with a fresh campaign against tax avoidance and evasion.