Boris Johnson's prorogation of Parliament was "unlawful" and "improper" the highest civil court in Scotland has ruled.
The three senior judges in Edinburgh said the prorogation was therefore "null and of no effect".
The government said it is "disappointed" by the decision and will appeal the ruling at the Supreme Court, adding proroguing Parliament was "legal and necessary".
No formal order will come before Tuesday, when the Supreme Court will hear the case.
The ruling comes a day after the prorogation took place in the early hours of Tuesday, with Parliament now suspended for five weeks.
A legal bid to challenge the suspension of Parliament by a cross-party group of 70 parliamentarians was initially rejected at the Court of Session, but the appeal was successful.