Pro-EU MPs welcomed the decision, which presents a serious obstacle to proposed Brexit plans.
Johnson conceded that a Brexit deal could take longer than two years to make.
The challenge to the policy comes from a group of citizens including investment manager Gina Miller.
Highlights of prime minister's speech at the Conservative Party Conference.
Foreign minister doesn't want the process to 'drag on' but EU exit will not begin this year.
The British prime minister doesn't yet have the answers to the big questions raised by Brexit.
May has the benefit of novelty and a national mission. But if she's not careful optimism will turn to impatience in the U.K.
Mishcon de Reya claims that Article 50 cannot be triggered without a full debate and vote.
The Conservative leadership frontrunner said the U.K.'s stance must be clear before negotiations.
One of the biggest questions is whether U.K. Prime Minister David Cameron will resign.