Len McCluskey has said Boris Johnson should be put under citizen's arrest after his suspension of Parliament was ruled unlawful by a Scottish court.
The Unite general secretary was quoted on Sky News as saying: “My advice to the Prime Minister is don’t go up to Scotland - you’re liable to face a citizen’s arrest.”
It comes as the Prime Minister's decision to suspend Parliament was ruled "improper" and "unlawful" by three senior judges in Edinburgh, who concluded it had been done with "the purpose of stymying Parliament".
The case, brought by a cross-party group of politicians, is now expected to be challenged in the Supreme Court by the Government which said it is "disappointed" by the decision.
The Prime Minister suspended Parliament for five weeks on Monday.
It is due to resume on October 14, days before the October 31st deadline for Brexit.
Mr McCluskey, a key ally of Jeremy Corbyn, has previously said Labour must support a pragmatic Brexit deal, saying it appeared to be impossible to stop no deal and that there was no path to a second referendum.
He said a no-deal Brexit would be “almost impossible to stop” and said he was not certain a no-confidence vote in Mr Johnson would be successful.
Shadow Brexit secretary Keir Starmer described the decision by the court as a “powerful judgement.”
Labour's shadow attorney general Shami Chakrabarti said: "This ruling shows that, despite what Boris Johnson has spent his privileged life thinking, he is not above the law.
"Labour will not allow his elitist shutdown of Parliament to enable him to dodge scrutiny and force through a disastrous no-deal Brexit."
