Boris Johnson from pushing through a no-deal Brexit against the will of parliament, Keir Starmer has pledged.
The shadow Brexit secretary said that if the prime minister is unable to reach an agreement with Brussels by 31 October, he must comply with the act and seek a further delay.
In a speech at the Co-operative party conference in Glasgow, Starmer said: "If he can not or I would say will not – get a deal without a deal.
"If no deal is secured by this time next week, Boris Johnson must seek and accept an extension. That's the law. No ifs, no buts. And if he does not, we will enforce the law. Whatever it takes, we want to prevent a no-deal Brexit. "
Starmer dismissed suggestions that the prime minister could circumvent the law by saying that he had a request for an extension with a second letter to the EU.
It's a ridiculous idea, "he said." That's the equivalent of attaching a post-it note to divorce papers saying 'only kidding'.
Officials are continuing to speak in Brussels over the weekend after the EU's chief negotiator, Michel Barnier, gave the go-ahead on Friday for intensive negotiations to start.
Starmer said that if the prime minister succeeded in getting an agreement, Labor would demand it.
The Labor MP and former director of public prosecution said that it would be "even worse" than Theresa May's rejected deal.
"No level playing field protections. No customs union. A green light to deregulate. That child of deal can never be one of labor supports, "he said.
Earlier on Saturday, Nicky Morgan said the prospects of a Brexit deal appeared to be "promising" after the meeting between Johnson and Ireland's Leo Varadkar.