He said Britain must leave bloc, with or without a deal, on Oct. 31.
The Court of Session in Edinburgh upheld an appeal against a lower court's ruling last week, saying Johnson's advice to Queen Elizabeth II that Parliament should be suspended was "unlawful because it had the purpose of stymieing Parliament."
The court said it will "make an order declaring that the prime minister's advice to HM the Queen and the prorogation which followed thereon was unlawful and is thus null and of no effect."
"We believe that the effect of the decision is that parliament is no longer prorogued," tweeted Jo Maugham, a lawyer in the Scottish case against prorogation.
Maugham said he expected the Supreme Court in London to begin hearing an appeal by the government on Tuesday.
"We are disappointed by today's decision, and will appeal to the UK Supreme Court," Downing Street said in a statement.
Scottish National Party (SNP) lawmaker Joanna Cherry, who led the Scottish legal case, told reporters outside the court that she was "absolutely delighted" by the ruling.
"Now, for every moment Parliament remains prorogued, the British government are breaking the law," Cherry said. "We, as politicians, are calling for Parliament to be recalled."
Cherry said she expected the Supreme Court to sit for three days to examine the Scottish case and two similar challenges in English and Northern Irish courts to Johnson's decision to suspend Parliament.
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