The “No” vote in the UK’s recent referendum on membership of the European Union has, in itself, plunged the UK into political, economic and social turmoil – even though the referendum was purely advisory (not legally binding on the government), and even though no political decision on a British exit has been taken, much less an exit effected in practice.
A lot has been written in analysis of the constitutional problems raised by the vote and its political fall-out. The clearest example to date is probably the piece by David Pannick QC in the Times: http://www.thetimes.co.uk/article/why-giving-notice-of-withdrawal-from-the-eu-requires-act-of-parliament-dz7s85dmw
Even that piece, though, is quite detailed and involved. I had to read it several times before I could state it simply enough to believe that I had understood its logic myself – so I thought the resulting summary might be helpful to others. Here it is:
1. Invoking Art.50 commits the UK, irreversibly, to a process which requires the amendment of existing UK statutes.
2. Existing UK statutes can only be amended by Act of Parliament*.
3. Therefore, Art.50 cannot lawfully be invoked without an Act of Parliament (and consequently a parliamentary vote).
Conclusion: it would be unlawful for the Prime Minister to invoke Art.50 without putting that decision to the vote in parliament.
* And crucially, the PM may not revoke (“frustrate”) a prior parliamentary decision by invoking the “royal prerogative”.