The Government has been accused of being "spiteful" after unveiling controversial plans to abolish a system where workers pay their union contributions through the payroll.
Ministers said the so-called check-off practice was "out-dated", describing it as a taxpayer-funded administrative burden on taxpayers.
But unions warned the move will damage industrial relations, describing it as a "political attack" on workers.
Gail Cartmail, Unite assistant general secretary said: "This is another spiteful measure from the Conservatives at a time when working people need unions like never before.
"It is a crude attempt to starve trade unions of money, money that is then used every day to promote training, workplace safety and hold up decent pay for millions of working people throughout the UK.
"This is nothing other than unnecessary interference by a government that has not got a clue about the reality of working life and the vital role unions have in workers' lives.
"This government will suffer a backlash from this, for people will see this for what it is - another needless, malicious attack on the people who are the backbone of our public services.
"For female low paid public sector workforce - health visitors, carers, cleaners and cooks - this is now the triple whammy. On top of the pay cap and end of working family tax credit, they now have to contend with this attack on their union.
"The Conservative party's attempt to pose as the party of working people is all the more laughable today than ever."
TUC assistant general secretary Paul Nowak said: "If payroll payment for union membership was out-dated as ministers claim, it would not be popular with so many of the UK's biggest private companies with positive union relations. This is really a political attack on union members from a government that is determined to re-balance power in the workplace so that workers lose their voice and their rights.
"Instead of going out of their way to poison industrial relations, the government should engage positively with workers and their representatives for the good of public services and the economy."
The Government said removing check-off will be included in its Trade Union Bill as part of "curtailing public cost of facility time subsidies."
Cabinet Office minister Matthew Hancock said: "In the 21st century era of direct debits and digital payments, public resources should not be used to support the collection of trade union subscriptions.
"It's time to get rid of this outdated practice and modernise the relationship between trade unions and their members. By ending check off we are bringing greater transparency to employees - making it easier for them to choose whether or not to pay subscriptions and which union to join."
Check-off has already been removed by a number of government departments including the Home Office and Ministry of Defence.
A spokesman for the Public and Commercial Services union said: "It's a totally unnecessary and vindictive move because it doesn't apply to any other salary based subscriptions like payments to charities.
"The experience in the civil service is that it's been hard work re-signing members to pay by direct debit, because ministers imposed arbitrary politically driven deadlines, but we will actually emerge stronger because we'll know more about our membership and be better organised in workplaces."
Unison general secretary Dave Prentis said: "This latest malicious manoeuvre from ministers shows how far they are prepared to go to deny nurses, teaching assistants, hospital cleaners and town hall staff a voice at work.
"Anyone a job in the public sector must wonder why the Government dislikes them so much that it is set on a series of such spiteful attacks on them through their unions.
"Allowing union subs to be taken directly from peoples' salaries is convenient for individual employees, their unions and their employers - and it works well in both the public and private sectors of the economy.
"In much of the public sector, unions pay the hospital trust or the local council the cost of taking the subs at source, so there is no cost to the public purse.
"Employees in workplaces where there are unions are more likely to earn more, be better trained, and have safer working conditions than those where there is no union.
"The Government is showing, once more, that by making it harder for unions in the public sector to stand up for their members, it can never be the party of working people. Attacks on unions are attacks on working people too."
Jon Ashworth, shadow cabinet office minister, said: "The Government should be focused on improving workers' rights and avoiding dispute, not mean spirited and ideological attack on the rights of ordinary trade union members in the workplace.
"Trade union members have the right to organise in the work place and this measure is wholly aimed at undermining that right.
"Labour is committed to check-off and the Tories should abandon plans to scrap it."