Liz Truss has been accused of "picking a fight with working people" to distract from Tory failures with plans to tear up key employment rights.
TUC General Secretary France O'Grady told the new Prime Minister "Hands off our hard won rights" - following reports she plans to axe EU protections, including the 48-hour working week.
Unions are even prepared to take the Government to court if Ms Truss shakes up rules to make it more difficult to strike or changes legislation on working hours, sick pay or health and safety issues.
A new poll for the TUC today found a majority of voters - including Tory supporters - back protecting workers' rights, such as paid holiday, rest breaks and safe limits on working time.
The survey of 3,000 adults found 79% want to keep all rights retained since Brexit, rising to 81% among people who said they will vote Conservative at the next election.
Some 74% support ending ‘ fire and rehire’ - rising to 77% among Tory voters. The poll also found 68% want to ban zero hours contracts, with around 66% of Conservatives support this.
In an interview with the Mirror, Ms O'Grady said: "We're sending a warning - hands off our hard won rights.
"What's so stark in this poll is it's people across the political spectrum. It doesn't matter which way they vote, they want action on zero hours contracts, they want action on fire and rehire, they want action on fair pay agreements.
"The very idea that Liz Truss, who believes there isn't enough hard graft in Britain - even though we work the longest hours in Europe - the very idea that they want to attack those rights that put safe limits on working time... is sickening when people are having such a tough time.
"She should be focused on making sure that people have enough money to pay their bills this winter."
As union chiefs prepare to head to Brighton for their annual conference next week, the TUC boss urged Ms Truss not to be a "P&O Prime Minister" in reference to the ferry firm which sacked nearly 800 staff in favour of foreign agency workers.
She said: "She wants to divide working people against each other and distract from what has been a massive failure of the Government she's been a part of... her fingerprints are all over policies that hurt working families.
"It won't wash. As the poll shows, people know the balance of power has shifted too far against working people."
She said the proposals should "be put in the dustbin of history" and warned the Government that public attitudes towards strike action have shifted as an autumn of walkouts looms.
"People know when they've been mugged off," she said.
Ms O'Grady also took aim at the new PM's threats to crack down on unions.
Ms Truss vowed to clip the wings of trade unions by introducing legislation in her first 30 days to raise thresholds for ballots and introduce minimum service levels during strikes.
But the TUC chief said the right to strike was a "fundamental British liberty", adding: "Nobody should be under any doubt that the purpose of the new law that Liz Truss is proposing is to make it harder for workers to stand up for our rights."
Ms O'Grady joked that Ms Truss' likely Business Secretary Jacob Rees-Mogg could be "our best recruiting sergeant" for trade unions.
She said: "Jacob Rees-Mogg has been pretty transparent about his agenda, that he wants to tear up workers rights and we are ready for it."
