How is the UK Supreme Court able to void the prorogation of the UK Parliament?
<p>In the UK, in practice, the government has power, but in theory the power lies in the hands of the Monarch, who acts on the advice of her Ministers.</p>

<p>The Prime Minister <em>advised</em> the Queen to prorogue Parliament, and she duly did so. The Supreme Court has since found that that advice was unlawful. Therefore, they say, Parliament has <em>not</em> been prorogued.</p>

<blockquote>
  <p>“This court has already concluded that the prime minister’s advice to Her Majesty was unlawful, void and of no effect. This means that the order in council to which it led was also unlawful, void and of no effect and should be quashed. This means that when the royal commissioners walked into the House of Lords it was as if they walked in with a blank sheet of paper. The prorogation was also void and of no effect. Parliament has not been prorogued. This is the unanimous judgment of all 11 Justices.”</p>
</blockquote>

<p>How, though, does this jibe with the legal fiction that Her Majesty has the power to prorogue Parliament whenever she so wishes, and that the Prime Minister merely <em>advised</em> her to do so, having no power to <em>order</em> her? If his advice was unlawful, surely her action in response to that advice was nonetheless lawful, as the fiction is that she acts in her own right, not merely as an automaton controlled by her Prime Minister.</p>

<p>The sovereign has the power to prorogue parliament at will, but actually exercises the power only on the advice of the PM (actually, according to paragraph three of <a href="https://www.supremecourt.uk/cases/docs/uksc-2019-0192-judgment.pdf" rel="noreferrer">the decision</a>, it is exercised on the advice of the privy council).</p>

<p>Other of the sovereign's powers are delegated to the courts.  The court is not unfamiliar with exercising the sovereign's power to void her own acts, as acts of parliament and acts of government ministers are also nominally acts of the sovereign.  While the courts cannot void acts of parliament, they can indeed void executive acts.</p>

<p>Two relevant quotes follow.  From paragraph 30, relating to the sovereign's lack of discretion in exercising the prerogative to prorogue parliament:</p>

<blockquote>
  <p>It is not suggested in
  these appeals that Her Majesty was other than obliged by constitutional convention
  to accept that advice. In the circumstances, we express no view on that matter. That
  situation does, however, place on the Prime Minister a constitutional responsibility,
  as the only person with power to do so, to have regard to all relevant interests,
  including the interests of Parliament.</p>
</blockquote>

<p>From paragraph 32, on the justiciability of questions concerning prerogative powers:</p>

<blockquote>
  <p>... political controversy did not deter the courts
  from holding, in the Case of Proclamations (1611) 12 Co Rep 74, that an attempt to
  alter the law of the land by the use of the Crown’s prerogative powers was unlawful.
  The court concluded at p 76 that “the King hath no prerogative, but that which the
  law of the land allows him”, indicating that the limits of prerogative powers were
  set by law and were determined by the courts.</p>
</blockquote>

united-kingdom