ITV News Deputy Political Editor Chris Ship reports
The ruling states the Government requires Parliament's consent to start the UK's withdrawal from the European Union.
The Government are set to appeal the decision in the Supreme Court just before Christmas.
This would then force the Government to put Article 50 legislation to the Lords and the House of Commons.
Sources from 10 Downing Street told Robert Peston the Prime Minister hands "will not be tied" by attempts by opposition parties to amend the Bill and she will fight any attempts to amend that legislation.
As a consequence, if Mrs May were to lose in the Lords and Commons, she will have "no choice but to go the country and seek a mandate in an early general election".
The future of Brexit now rests with the Supreme Court - the highest court in the land.
It came after three judges unanimously ruled Theresa May does not have the power to bypass MPs by relying on the prerogative to trigger Article 50 of the Treaty on European Union and begin the two-year period of divorce negotiations with the 27 other countries in the EU bloc.
The Lord Chief Justice, Lord Thomas, said to do so would be contrary to "the fundamental constitutional principles of the sovereignty of Parliament".
The court rejected the Government argument that prerogative powers were a legitimate way to give effect "to the will of the people" who voted by a clear majority to leave the European Union in the June referendum.
It is widely expected that 11 Supreme Court justices will sit between December 5 and 8 to hear what is one of the most important constitutional cases in generations.