The panel of three judges ruled in favour of the challenge brought by investment manager Gina Miller, which stated that Mrs May did not have the right to bypass Parliament and begin the process of secession from the European Union without MPs' consent.
Mrs May had planned to invoke Article 50 by March next year and had promised MPs would be allowed to debate the exit plans but would not be allowed to vote on the timing of secession.
Government lawyers had argued prerogative powers were a legitimate way to give effect "to the will of the people" who voted to leave the European Union in the June referendum.
Unless overturned on appeal at the Supreme Court, the ruling threatens to plunge the Government's plans for Brexit into disarray as the process will have to be subject to full parliamentary control.
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."
The Government wasgiven the go-ahead to appeal against the ruling at the Supreme Court, and it has since been announced that the case will be heard in December.
A statement from the Government after the verdict said it was "disappointed" but was determined to respect the result of the referendum.
Read more about the High Court ruling via the following links: