Widespread strike action could run into the Spring if hundreds of thousands of workers vote for walkouts in looming ballots.
PCS General Secretary Mark Serwotka warned that disputes over pay and working conditions could run for "months" - and said people were fed up of the Government's "callous, brutal dismissal" of workers' concerns.
As Liz Truss's Government was plunged into deeper crisis, union leaders in Brighton supported a bid to coordinate strikes for maximum impact.
Essential workers across the economy, from nurses and midwives to teachers, will vote in ballots in the coming months for what could be the largest bout of industrial action in more than decade.
Railway workers, council employees, barristers, lecturers and dockers have already taken strike action this year, with the threat of further walkouts looming.
In an interview with the Mirror, Mr Serwotka said cost of living pressures had become "existential" and the Government was forcing the least well off to pay the price for its mess.
The PCS union, which represents civil servants, is asking 150,000 members in 214 Government departments to go on strike, demanding a 10% pay rise and £15-an-hour minimum wage.
Public sector pay offers vary between around 3% to 5% but fall below inflation at 10%.
Drivers’ licences, passports, courts and benefit appointments could grind to a halt before Christmas if the ballots are successful.
The PCS will announce the results of its ballots next month and unions are in talks about coordinating stoppages to put the Government's feet to the fire over pay and working conditions, Mr Serwotka said.
“If the RMT are on strike, well that’s probably a good time for our members in the Department for Transport to go on strike because if they people don’t use the rail, they use the roads," he said.
"We have got members in DfT on poverty pay. They might think that they’ll have more effect joining forces on the same day.
“None of us want it but I have never seen such a callous, brutal dismissal of real concerns than at the moment… they are telling the people who can afford it the least that they have to pay the price of all this mess they made.”
The PCS boss added: “I don’t think anyone thinks this can be won quickly. I think these are discontents that are going to go on over many months - unless the Government completely does another U-turn and says, ‘ok we realise the importance of workers’.
“In that instance, I think what we will be trying to do is plan for a long-ish campaign over the winter, into the Spring.”
He said there was a "critical mass" among unions, adding: "This is going to grow and grow."
“At what time and when that reaches everybody remains to be seen but the Government needs to understand they are not just dealing with a protest.”
PCS has presented the Department for Work and Pensions (DWP) with a dossier of the grim challenges its members in the department are facing.
“It’s existential for some people,” Mr Serwotka said. “The testimonies we are getting from members are devastating, about not putting the lights on, using candles, skipping meals, not eating properly."
He added: “I haven’t seen anything like this - because it comes on the back of 10 years of austerity where we have already seen huge cuts to the bone.
"People are working second, third jobs. It is a crisis of monumental proportions.”
A DWP spokesperson said: “We recognise that people are struggling with rising prices which is why we’ve brought forward a huge package of support for families worth £37bn including providing all households with £400 towards energy costs.
“This year’s pay award was set at the highest level permitted within the Civil Service guidance, recognising the vital importance of public sector workers while providing value for the taxpayer.”
