An application for a judicial review of the decision to prorogue Parliament was made by Gina Miller and heard at the High Court in London on 6 September 2019. The judgement was published on 11 September 2019.
R (Gina Miller) v The Prime Minister  EWHC 2381
The court concluded that the claim, that the advice given was unlawful and an abuse of power, is not capable of being determined by the courts (i.e.the claim is non justiciable).
The court also certified pursuant to section 12(3A)(c) of the Administration of Justice Act 1969 that a sufficient case for an appeal to the Supreme Court has been made out to justify an application for leave to bring such an appeal.
The appeal has been scheduled for a hearing in the Supreme Court next Tuesday, 17 Sep 2019.
An extract from the summary expands on the reasons for its decision:
Both the decision of the Prime Minister that Parliament should be prorogued at the time and for
the duration chosen and the advice given by the Prime Minister in the present case were inherently
political in nature and there are no legal standards against which to judge their legitimacy. 
There is no legal measure of the length of time between Parliamentary sessions, nor even a
constitutional convention which governs that matter, albeit constitutional conventions are not
Parliament may be prorogued for various reasons, including political reasons, and
there is no statute, law or convention which requires Parliament to sit in constant session. 
If the primary purpose of prorogation is to undertake preparations for the Queen’s Speech, it would
be impossible for the court to decide whether the period of prorogation is excessive as it would
require the court to examine and assess how much time it is legitimate for the Government to
spend on its preparations in relation to each aspect of its proposed legislative programme, a purely
political matter. 
It is also impossible for the court to assess by any measurable standard how much time is required
to hold the Government to account, including by passing legislation that would require the Prime
Minister to take steps to avoid leaving the European Union without an agreement. This is
graphically illustrated by the speed with which the European Union (Withdrawal) (No 6) Bill has
been enacted. The ability of Parliament to move with speed when it chooses to do so undermines
the underlying premise of the case for the claimant that prorogation would deny Parliament the
opportunity to do precisely what it has now done. 
A full summary is available at
and the Full Judgement at
(local copies are also available)