On what basis is Theresa May triggering Article 50 (Brexit) if the Supreme Court said she couldn't?
Just today (March 21st 2017), the news came out around the web that Theresa May will trigger Article 50 (Brexit) on March 29th:
The UK will begin official divorce proceedings with the European Union on March 29, prime minister Theresa May's spokesperson has announced. — Ars Technica
However, it is my understanding that the Supreme Court asserted Theresa May was not allowed to do this (warning: there's an autoplaying video on that page), and that it was a matter for the entire parliament to decide.
In a joint judgment of the majority, the Supreme Court holds that an Act of Parliament is required to authorise ministers to give Notice of the decision of the UK to withdraw from the European Union
What's going on? Did something occur in the meantime that reversed this decision? Is Theresa May attempting to get parliament's agreement on that date, or is something else going on?
I believe that she passed it while still complying with the Court's decision - that is by not, to quote the article, "bypassing MPs and peers". Writing this as a comment because origimbo's answer is basically correct.
The UK supreme court does not say May can't do it, it says she can't do it without MP approval. She got MP approval, so now she can do it.
Important comment was that there was at least a theoretical possibility of the Brexit bill failing or being drastically modified by the democratically elected representatives. But I really don't know why May bothered to fight the court case rather than do it in this traditional Parliamentary manner.
(the executive/legislature distinction is often unclear due to strong party vote discipline, but EU membership is effectively UK constitutional law (see "Factortame") and Brexit is really putting stress on load-bearing bits of constitution)
Choosing March 29th seems odd. Surely it would have been more dramatic to go for March 25th - the 60th anniversary of the signing of the Treaty of Rome. But I suppose nobody on board the Brussels gravy-train would be in the office on a Saturday!
@alephzero Doing it on the anniversary would widely be regarded as an unfriendly act. You don't want to do that to a party you will need to negotiate with about quite serious matters.
@origimbo's answer is completely correct and accurate. I would just like to go into the details of the Supreme Court ruling and what the UK Parliament has done.
Supreme Court ruling
Basically, the Supreme Court has ruled that as seen in this article by The Telegraph.
Supreme Court justices ruled, by a majority of eight to three, that Prime Minister Theresa May cannot lawfully bypass MPs and peers by using the royal prerogative to trigger Article 50 of the Lisbon Treaty and start the two-year process of negotiating the UK's divorce from its EU partners.
[ ... ]
A one-line Bill is expected to be drawn up and debated in Parliament, and Mrs May will be hoping that if that is the case, the Bill can go through the Commons and the Lords swiftly before becoming law as an Act of Parliament.
Actions taken by the government
So, the government has complied and Prime Minister Theresa May published the Brexit bill on 26 January 2017.
1 Power to notify withdrawal from the EU
(1) The Prime Minister may notify, under Article 50(2) of the Treaty on European Union, the United Kingdom’s intention to withdraw from the EU.
(2) This section has effect despite any provision made by or under the European Communities Act 1972 or any other enactment.
The bill was then passed in both Houses of Parliament unamended for enactment by royal assent, which was given on 16 March 2017.