The shadow Brexit secretary declared, "I'd better get back to London", when he was informed after a speech to the TUC in Brighton that Scotland's highest court had ruled the prorogation to be unlawful.
In a unanimous decision in Edinburgh’s Court of Session, three judges ruled that the prorogation was unconstitutional, but no order was given to reverse it ahead of a full hearing at the Supreme Court on Tuesday.
Sir Keir vowed to "get Boris Johnson back in parliament" to hold him to account over Brexit.
He told delegates: "It was obvious to everybody that not only was shutting down parliament at this crucial time obviously, the wrong thing to do, we should be sitting each and every day to resolve this crisis.
- Read more
"But that the prime minister was not telling the truth about why he was doing it."
Sir Keir added: "This is really important. The idea of shutting down parliament offended people across the country and then they felt they weren’t being told the truth - and that sums up the man.
"I’m really pleased with this result. I’m surprised because for a court to make a declaration like that on an issue like this is a huge thing for us.
"It vindicates everything we’ve done last week and I think what I can do and what others need to do is get back to parliament, see if we can’t open those doors and get back in and get Boris Johnson back in parliament so we can hold him properly to account."