Liberal Democrat leader Tim Farron welcomed the news, adding: "It is disappointing that this Government was so intent on undermining parliamentary sovereignty and democratic process that they forced this decision to be made in the court, but I welcome the news today that MPs will get to vote on the triggering of Article 50. The government argued that the prime minister can trigger Article 50 using royal prerogative powers and that parliamentary consent was not required following the clear public referendum vote in favour of leaving the EU. But the government was going to contest that view in an appeal, and said the referendum was held only following "a six-to-one vote in the Commons to give the decision to the British people". The court looked at examples ranging from the 1600s to the 1970s Laker Airways legal battle

Parliament had a vote on the UK joining the European Union back in the 1970s, so there is no convention of the Royal Prerogative being used in legislation relating to the European Union

Allowing MPs a vote on the final Brexit deal at the end of the negotiations would not amount to parliamentary approval because once Article 50 is triggered there is no way that the UK will not leave the EU, and in doing so existing laws will be changed

David Davis points out that MPs voted by six to one for the referendum to be held, but the judgement says that the referendum bill, and background briefings, made clear that the referendum was advisory rather than mandatory. International Trade Secretary Liam Fox said the Government is "disappointed" at the High Court decision on Article 50 adding "the Government is determined to respect the result of the referendum". In a shock move, three senior judges have ruled the Prime Minister does not have power to trigger Brexit despite the referendum decision. But the lord chief justice declared: “The government does not have power under the crown’s prerogative to give notice pursuant to article 50 for the UK to withdraw from the European Union.”

May’s spokeswoman said the prime minister was working from Downing Street as usual and would not be making any public statement on Thursday about the government’s defeat in court. Some in Westminster believe the high court decision has made the prospect of an early general election more likely by increasing the risk that the prime minister will be unable to manage the tensions within her own party . "I now fear that every attempt will be made to block or delay the triggering of Article 50," he said.
The High Court rules that Parliament must vote on whether the United Kingdom can start the process of leaving the European Union by triggering Article 50 of the Treaty on European Union.