Donald Trump’s executive orders banning immigration from seven Muslim majority countries and imposing an indefinite stay on refugees fleeing the humanitarian catastrophe in Syria has caused untold misery and legal uncertainty for tens of thousands.
Mr August Flentje, lawyer for Trump’s justice department, stated that this exercise of executive power was ‘perfectly constitutional’, though the US courts halted the ban in its initial form.
As many of us in this country look on with horror at what has unfolded in the US, it is somewhat comforting to be reminded, in the wake of the Supreme Court’s Brexit decision, that the nature of UK constitutional law means such an exercise of executive power would almost certainly be unsuccessful here.
The initial refusal of the government to allow parliament a say on the triggering of Article 50 led to the Brexit case R (on the application of Miller and another) v Secretary of State for Exiting the European Union  UKSC 5. The case gives the Supreme Court’s judicial analysis into the exercise of executive power in British constitutional law.
Going back to basics, the Supreme Court cited the comments of the great Edwardian jurist A.V Dicey, stating that the UK, by virtue of its unwritten constitution, was the most ‘flexible polity in existence’; capable of evolving in response to events, legislation and conventions in a ‘pragmatic and principled way’, However, Dicey noted that the one fundamental assumption that underpins the unwritten constitution is that of Parliamentary sovereignty; Parliament alone has “the right to make or unmake any law whatsoever… no person or body is recognised by the law as having a right to override or set aside the legislation of Parliament”.
The Supreme Court also cited the early 17th century Case of Proclamations [1610 ] EWHC KB J22,in which CJ Edward Coke declared by way of challenge to the increasingly absolutist government of the Stuart monarch King James I that ‘the King by his proclamation or other ways cannot change any part of the common law, or statute law, or the customs of the realm’.
By the end of the 17th century, this position was firmly entrenched in law, and the Supreme Court specifically identifies a number of laws passed during this time as setting down the constitutional framework for the separation of powers as they currently exist. ‘A series of statutes enacted in the twenty years between 1688 and 1707 were of particular legal importance. Those statutes were the Bill of Rights 1688/9 and the Act of Settlement 1701 in England and Wales, the Claim of Right Act 1689 in Scotland, and the Acts of Union 1706 and 1707 in England and Wales and in Scotland respectively.’
These statutes formally recognise not just the sovereignty of Parliament but the independence of the judiciary – that it is the judiciary’s role to uphold, not just the rule of law by applying the law to the cases that come before it, but to develop the law themselves in accordance with the common law, insofar as this is not inconsistent with statute.
In the Miller judgment, the Supreme Court points out that the Crown’s administrative powers are now of course exercised by the executive but exercise of that power must still be consistent with these 17th century principles. This effectively means that the executive cannot exercise its power in a way which has the effect of creating laws.
A classic statement of this position was given in the case of The Zamora  2 AC 77 cited with approval by the Supreme Court in Miller:
‘The idea that the King in Council, or indeed any branch of the Executive, has power to prescribe or alter the law to be administered by Courts of law in this country is out of harmony with the principles of our Constitution…. No one would contend that the prerogative involves any power to prescribe or alter the law administered in Courts of Common Law or Equity.’
In Miller, the Government did not seek to challenge these well-established principles, but instead sought to argue that triggering Article 50 was part of the ‘residual’ power vested in the Crown to enter into and leave international treaties, known as the Royal Prerogative.
The majority in Miller, however, disagreed.
They cited the classic definition of the royal prerogative as described by Lord Reed in Burmah Oil Co (Burma Trading) Ltd v Lord Advocate  AC 75, that royal prerogative is a source of power which is “only available for a case not covered by statute”.
‘The residual prerogative is now confined to such matters as summoning and dissolving Parliament, declaring war and peace, regulating the armed forces in some respects, governing certain colonial territories, making treaties (though as such they cannot affect the rights of subjects), and conferring honours.’
The Lords went on to cite a series of authorities (De Keyser and Fire Brigades Union) suggesting that as the prerogative in the UK is a residual prerogative, it will always be ‘displaced in a field which becomes occupied by a corresponding power conferred or regulated by statute’.
On the facts of Miller, the majority found that the effect of European Communities Act 1972 was precisely to bind future parliaments in a way which is ‘inconsistent with the future exercise by ministers of any prerogative power to withdraw’ and thus displaces the prerogative.
The takeaway from Miller is even in cases where the executive apparently has prerogative to act in a certain way, this residual prerogative can be displaced by any suggestion that it is contrary to parliamentary intention. Given the residual, exceptional nature of the prerogative, an executive order such as the one introduced by the Trump on the US would almost certainly be displaced by even a whiff of it being contrary to either statute or common law.
It was truly heartening to see the judiciary hold firm in the face of relentless assault on these fundamental principles by the right-wing media and populist politicians, and to see them defend their independence that in ordinary times we would have taken for granted.
The subsequent speed with which the Brexit Bill passed through Parliament, and the fact Article 50 was triggered only marginally later than planned on 29th March 2017, may to some make the Miller case seem like a storm in a teacup or a huge waste of time and money. However, the principle upheld in Miller is of fundamental importance and the decision reached, whatever has happened since, fires a very important warning shot across the boughs of UK’s current and future political leaders, tempted to use the prerogative to make vast constitutional changes without consulting parliament.