Boris Johnson's decision to suspend parliament was unlawful, the Supreme Court has ruled, BBC reported.
Mr Johnson suspended - or prorogued - parliament for five weeks earlier this month, but the court said it was wrong to stop MPs carrying out duties in the run-up to Brexit on 31 October.
Supreme Court president Lady Hale said, "the effect on the fundamentals of democracy was extreme".
Downing Street said it was "currently processing the verdict".
Mr Johnson argued he wanted to carry out the prorogation ahead of a Queen's speech so he could outline his government's new policies.
But critics said he was trying to stop MPs from scrutinising his Brexit plans.