Britain's Supreme Court said today that rulings from a secretive UK spy tribunal can now be appealed against after a legal challenge from pressure group Privacy International.

Decisions of the Investigatory Powers Tribunal (IPT), which rules on legal cases involving surveillance powers and the British spy agencies (MI5, MI6 and GCHQ), were previously immune from being appealed against. If you didn't like the IPT's verdict, tough – there was no way of asking a more senior judge to take a second look at it.

Section 67(8) of the Regulation of Investigatory Powers Act meant that, unlike any other law court in the UK, decisions of the IPT "shall not be subject to appeal or be liable to be questioned in any court".

Lord Carnwath, one of the Supreme Court judges who delivered the majority verdict of the seven-strong judicial panel, ruled today that even with that legal wording, decisions that were "legally invalid" could still be questioned. The common law, he said, has a strong presumption against "ouster", which is the legal idea of stopping the High Court from overturning junior courts' judgments by having its own judges review them.

"I am unimpressed by arguments based on the security issues involved in many (though not all) of the IPT's cases," said Lord Carnwath. "As this case shows, the tribunal itself is able to organise its procedures to ensure that a material point of law can be considered separately without threatening any security interests."

The judge also commented in an aside in the 113-page judgment that MPs cannot make laws that stop the High Court from enforcing the law: "Parliament cannot entrust a statutory decision-making process to a particular body, but then leave it free to disregard the essential requirements laid down by the rule of law for such a process to be effective."

Privacy International barrister Dinah Rose QC successfully argued that, contrary to what the spy agencies' lawyers said, the ordinary courts have plenty of ways of protecting secret and sensitive material when carrying out judicial reviews.

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Megan Goulding, a lawyer working for PI's fellow pressure group Liberty, which intervened in support of PI, said in a statement: "Putting oversight of the intelligence agencies – with their sweeping intrusive powers under the Snooper's Charter – beyond the review of ordinary courts, is not just undemocratic, but a sinister attempt to reduce the safeguards that protect our rights."

What kicked the whole case off was Privacy International starting a legal challenge against GCHQ hacking, which Britain's lax laws allow the spy agency to do more or less whenever and to whomever it pleases with few meaningful controls. When PI lost in the IPT, they tried to take it to judicial review in the High Court, only for the spy agencies to pull out the section 67(8) trump card. Both the High Court itself and the Court of Appeal agreed with PI.

Dissenting from Lord Carnwath were Lords Sumption and Wilson, who said section 67(8) was clear and that Parliament had obviously intended to ensure the IPT could not be judicially reviewed or otherwise appealed against.

You can read the full judgment here (PDF). ®