SCM Investments founder Gina Miller has won a Brexit legal challenge that paves the way for MPs to vote on triggering Article 50.
The court judgement says the Government’s argument is contrary to the “fundamental constitutional principles of the sovereignty of Parliament”.
The Government has already confirmed it will appeal.
If it loses, the triggering of Article 50, scheduled for Q1 2017, could be delayed.
The pound surged 1 per cent to $1.24 against the dollar immediately following the decision, then pared back slight.
The domestically sensitive FTSE 250 has also made slight gains.
In a statement read out following the decision, Miller said the legal challenge was about “process, not politics”.
She urged the Government not to appeal and instead have a “proper debate” on the UK’s exit from the European Union in parliament.
However, the Supreme Court was already on standby to give an expedited hearing next month on the expectation that regardless of today’s outcome an appeal was likely to be lodged.
The appeal date is expected to be between December 5-8.
Speaking to the House of Commons, Minister for International Trade Liam Fox said he is “disappointed” by the court’s decision.
Miller has been represented by law firm Mishcon de Reya.
FX & macro strategist at ETF Securities Martin Arnold says the judgement is likely to give “fresh impetus” to the pound.
“Optimism from investors is stemming from the fact that the UK Parliament will attempt to steer the UK further away from the ‘hard Brexit’ stance of the Conservative government, which will be less damaging for the UK economy.
However, he warns volatility is likely to remain due to the government appeal.