Cooper accused the Prime Minister of lying about Parliament “blocking Brexit” and said she “cannot believe a single word [May] says”.
“The Prime Minister yesterday said she was calling a general election because Parliament was blocking Brexit,” Cooper rallied at PMQs.
“But three-quarters of MPs and two-thirds of the Lords voted for Article 50. So that’s not true is it?
“And a month ago, she told her official spokesman to rule out an early general election - and that wasn’t true either, was it?”
She finished, to huge cheers from the Labour benches:
She wants us to believe that she is a woman of her word. But isn’t the truth that we cannot believe a single word she says?"
May got up to answer, but was met with a wall of noise from Labour MPs.
She was flanked on the Commons benches by Scottish Secretary David Mundell, who mouthed something in response to Cooper’s question.
After some speculation on Twitter that he could have been mouthing “ignorant bitch”, Mundell clarified his remarks:
When Theresa May finally did get up after he bruising, she accused Labour of “making it clear they were thinking of voting against the final deal” and the SNP of “having said they will vote against the legislation necessary to leave the EU”.
May also said the Liberal Democrats planned to “grind government to a standstill” and that the House of Lords have “threatened to stop us every inch of the way”.
Peers voted in March to pass the Bill allowing Theresa May to trigger Article 50, despite having its amendments dismissed by the House of Commons.
But Cooper’s challenge to the PM was widely hailed, with talk from pundits of it being a useful springboard into a leadership challenge if Labour is wiped out at the election of June 8.
Cooper stood for the leadership in 2015 but came third.
Jeremy Corbyn won with a landslide majority of 59%, while his challenger Andy Burnham trailed in second with 19%.
Cooper achieved 17% of votes by Labour members, while Liz Kendall was last place with 4.5%.