General secretary Frances O’Grady said the Prime Minister had torn up rules and was ready to defy the law.
Speaking ahead of the opening of the TUC Congress in Brighton, Ms O’Grady urged MPs to “call Boris Johnson’s bluff” and wait until October 31 so that an extension to leaving the EU is secured.
“Then let’s have that general election on our terms, not his. I won’t rest until the Article 50 has been secured beyond any doubt.
“We know what kind of man Boris Johnson is, and the people he gets to do his dirty work.
“We don’t trust him. He would sell livelihoods down the river because all he cares about is political power.”
Ms O’Grady said events in the coming days and months will shape jobs and living standards for a generation.
A no-deal Brexit would be a “disaster” for workers, especially those in the NHS, civil service, the food industry and ports, she added.
Ms O’Grady said: “A no-deal Brexit means higher fuel prices and a more expensive weekly shop, it will destroy good British jobs, less money for the NHS and medicine shortages for cancer patients.”
Leaving the EU without a deal would pave the way for a low tax, low regulation Britain that works for hedge fund managers but offers nothing for working people, she added.
TUC president Mark Serwotka told the Congress that Boris Johnson was “worse than Thatcher.”
He told delegates: “Johnson’s cabal of hard-right, free market fanatics will try to wreak havoc on this country and cause untold damage.
“His Cabinet includes a Home Secretary who believes that capital punishment should be brought back, and another member who believes that women should be denied the right to an abortion even in the case of rape.
“In any circumstances, a Johnson government would pose a massive threat to workers, unions and public services.
“The prospect of Johnson and his zealous accomplices controlling a no-deal Britain post Brexit - flogging off anything that isn’t nailed down to the highest bidder - is bleak.
“Now is the time for those who have been lukewarm to the Labour leadership to stop and think. Do you want five years of Boris Johnson in hoc to his mate Trump? If not we need to unite behind Jeremy Corbyn to deliver a radical alternative based on an end to austerity.
“We cannot allow the issue of Brexit to divide our movement. Boris Johnson will pretend he stands for the 52% who voted to leave, but he only represents the richest 1%.”