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Sir John Major has said he will seek a judicial review should Boris Johnson become Tory leader and suspend Parliament in order to deliver a no-deal Brexit. But, just what is a judicial review?

Every month the courts in England and Wales receive around 300 requests to test whether decisions and actions taken by government departments, local authorities and other public bodies are lawful.

The process is a tried and trusted legal procedure which helps hold policy-makers to account - but it can last a long time.

It takes about two months from an application being lodged to a decision as to whether to grant permission for a full court hearing, and then many more weeks or months before the hearing takes place, with, usually, a further delay before the judgment is given.

In judicial review cases, it's not the court's job to rule on whether the decision made by the public organisation is correct or not.

What the court is concerned about is the way in which the decision or action has been taken: have the right procedures been followed? Has the public body exceeded its powers?

Additionally, the court may be asked to rule that a decision or action is "irrational" - in other words, so unreasonable that no sensible person could have arrived at it.

Seldom do judicial review cases involve those of constitutional importance, as Sir John Major's would.

Media playback is unsupported on your device Media caption The former prime minister says it would be "unacceptable" to force through a-no deal

Only the Queen can decide to suspend Parliament, but it's highly unlikely there would be a legal challenge against any decision Her Majesty made because, as the eminent QC, Lord Pannick, points out, the head of the UK's constitutional structure is "immune" from the process.

So, as Sir John indicated on BBC Radio 4's Today programme, any judicial review would probably revolve around the legality of the advice to suspend Parliament offered to the Queen by Mr Johnson.

And that could turn on whether such advice is "irrational".

Writing in the Times last month, Lord Pannick said those bringing a judicial review case would need to show that the PM's advice breached a fundamental legal principle - the sovereignty of Parliament.

The cross-bench peer suggests the High Court would look favourably on such a challenge if it demonstrated the reasons for a suspension were to avoid Parliament making a decision of constitutional importance on an issue they had previously expressed opposition to.

Human rights lawyer Adam Wagner says British judges showed in the Article 50 case the importance they attach to the sovereignty of Parliament, when the UK Supreme Court ruled in January 2017 that MPs and Peers should be given a vote on triggering the Brexit process.

The Supreme Court, in a case which had begun with a judicial review hearing in the High Court, ruled: "It would be inconsistent with long-standing and fundamental principle for such a far-reaching change to the UK constitutional arrangements to be brought about by ministerial decision or ministerial action alone."

Mr Wagner suggests that, like the Article 50 case, if the High Court were to hear a challenge against Boris Johnson's advice to the Queen, the outcome - whichever way it went - would result in a 'leapfrog' appeal to the Supreme Court, which would be the final arbiter.

That raises the question as to whether all this could be completed before the Queen acted on Mr Johnson's request and brought Parliament to a halt in order for Britain to leave the EU.

Legal experts say it could just about be done - if a week was set aside for a series of urgent court hearings before the final exit was carried out.

But Jolyon Maugham QC, a barrister from the Good Law Project, says leaving it until October could be too late so he would like the issue to be clarified in advance - before any advice from the new PM is passed to the Queen.

Image copyright PA Media Image caption John Major says proroguing Parliament would put the Queen amidst 'a constitutional controversy'

Mr Maugham, who was behind a successful legal challenge at the European Court of Justice over the UK's power to revoke Article 50 unilaterally, is consulting a specialist constitutional lawyer about whether judicial review proceedings could be brought now.

Such a challenge might be based on statements already made by Boris Johnson about his intentions, he suggests.

However, judicial review proceedings carry no guarantees - particularly when they involve such sensitive matters that have not been tested in modern times.

As Mr Maugham says: "These are waters we haven't had to swim in as lawyers for centuries."