Jo Swinson, leader of the Liberal Democrats, said the judges’ decision proved Boris Johnson was “not fit to be Prime Minister”.
She said: “The court have found what we all knew all along, Boris Johnson has again proven he is not fit to be Prime Minister.
“This shutdown was an unlawful act designed to stop Parliament doing its job and holding the Government to account.
“Given this verdict, Parliament should be sitting so that we can continue to question the Conservative Government on their disastrous Brexit plans.
“This verdict has been unanimously agreed by experienced judges who have considered the case on its merits, acting as impartial guardians of our democratic system.
“The rule of law is an important pillar of our democracy, and those looking to use this opportunity as an excuse to attack these judges would be not only attacking them, but also the entire principle of our legal system.”