The Labour MP said the prime minister had behaved as if “only she should have a say on the way we leave the EU” when she fought in the courts to stop MPs having a vote on Article 50.
The Government lost its case in the Supreme Court on Tuesday, when judges upheld that triggering Article 50 required an act of parliament.
Umunna, who set up Vote Leave Watch to scrutinise Brexiter’s promises after the referendum, clashed with pro-Brexit Tory Kwasi Kwarteng on a Channel 4 News panel discussing the Supreme Court’s decision.
Umunna said: “Those who argued for us to leave the European Union, they did so on the basis that parliament, the voice of the people, should be sovereign.
“The prime minister, like some pseudo dictator, saying that only she should have a say on the way we leave the European Union...
“I accept there’s a mandate for the Government to take us out of the European Union but I’m not just sitting by while they do so in a way that’s going to trash the livelihoods of middle and lower income people in my constituency and do so in a way which disregards many of the promises that were made.”
He continued: “For example, a lot of people now who are watching this programme, expect £350 million per week extra to go to the NHS.
“We have a right, through parliament, to hold them to account for those promises and ensure that the way in which we leave the EU is in the British national interests and interests of the families.”
Kwarting questioned his use of the phrase “dictator” and said: “We had a vote to hold a referendum... Nobody said at the time that there was anyway would reverse that result... it would make a total mockery.”
The panel included Gina Miller, the lead claimant in the Supreme Court case, as well as Ukip’s Patrick O’Flynn and the SNP’s Stephen Gethins.
Flynn and Gethins argued over the referendum, with Flynn accusing Miller of being part of a group of “high-earning London households used to getting their own way”.
“How dare you presume you know who I am? That I’m the London elite?” Miller, who has received death threats over the court case, asked.
When O’Flynn tried to talk over her and say what “a lot of the country think” about wealthy Londoners, Miller said: “You know nothing about me so don’t make this personal. Talk about my case not about me.”
O’Flynn said: “You represent a certain strand of opinion on this.”
Miller said: “No, I don’t represent anyone but myself.”