A new report says the body of advisers dating back to feudal times can no longer escape constitutional reform.

A shaft of light was cast last week on the Privy Council, one of the most obscure and unknown parts of the British constitution and hidden parts of the justice system.

The law lords who sit as the Privy Council's Judicial Committee ruled that a gallantry medal established by the Queen for Trinidad and Tobago was unconstitutional because it discriminated against non-Christians.

But what is the Privy Council - and its court? A recent report by Justice, the law reform body, says that ?the lack of accessibility and transparency of the Privy Council means that almost no one understands its processes?.

The Council is a body of advisers to the Queen dating back to feudal times when the monarch ruled by divine right. Yet the Privy Council and its court are far from a relic of history.

Until recently, its members took precedent over other MPs whom the Speaker might call on to speak - and they could speak for longer. They also take precedence after Knights of the Garter and of the Thistle when it comes to ceremonial hierarchy.

More important, though, Privy Counsellors can make orders that bypass Parliament, while still having the same force as democratically passed laws. And their laws don't have even have to carry a statement that they comply with the European Convention on Human Rights.

Meanwhile, its court, the Judicial Committee, operates away from the public eye, in a little-visited room off Downing Street. But it has the power to save a person from death or, for instance, as in its recent ruling, to dispossess Chagos Islanders from their homes.

As Roger Smith, the director of Justice, put it in his forward to a Justice report: ?Many people may have heard of its judicial committee, but its other roles emerge from the constitutional fog only occasionally.?

The Privy Council?s continued existence, he added, is ?regarded as vaguely charming and largely formal?. But it can still display ?the power that once it had more widely as an instrument of feudal rule?.

The Justice report, a fascinating expose by Patrick O?Connor, QC, of Doughty Street Chambers, notes that even the statutory definition of the Privy Council is circular: it is defined as the members of ?Her Majesty?s Most Honourable Privy Council?.

The term was first coined under Henry V in the early 15th century; by then it had had jurisdiction to deal with finance, aliens and trade, the church and preservation of the King?s Peace. By 1487 its judicial function became the infamous Court of the Star Chamber, specialising in supervising the press and public morality.

Who are Privy Counsellors? Currently there are more than 540, mostly senior politicians who were once MPs. As with a gentlemen?s club or secret society, members swear allegiance to the Queen and to ?assist and defend...against all Foreign Princes?.

Their task is to advise the sovereign on the exercise of the Royal Prerogative, her personal powers, and carry them out on her behalf.

Patrick O?Connor says: ?Some of these powers are purely symbolic and ceremonial: some are of constitutional importance and high sensitivity. In the latter cases, the advisory role is often a fiction: the Privy Council is a vehicle for executive decisions by the Government, formally issued under the name of the monarch.?

The Lord President is always a Cabinet minister and normally also is leader of the House of Commons or the Lords. It is described as ?synonymous? with government. But there are many hundreds of Privy Counsellors, Mr O?Connor notes, who are nothing to do with the current government.

Once out of high office, no counsellor ? they are all appointed for life - is consulted or summoned to meetings, he says, ?save upon the sovereign?s demise or marriage?. And, he notes, its rapid expansion in recent decades leaves the impression, at least, that it is ?another resource for Government patronage?.

In 2007, a snapshot showed it met 13 times formally, on issues ranging from UN sanctions against North Korea to the charter of the Royal Caledonian Horticultural Society. There is no consistent rationale for these diverse functions, Mr O?Connor says. ?They are little more than a rag bag of historical accidents.?

As for the Judicial Committee, it is made up of the same 12 law lords who make up Britain?s highest court, namely the Appellate Committee of the House of Lords, occasionally supplemented by leading foreign judges.

It acts as the final court of appeal for many former colonies and UK overseas territories, mainly in the Caribbean but also including appeals from the Channel Islands and Isle of Man, Admiralty appeals from the Cinque Ports, and disciplinary appeals involving doctors and dentists as well as some appeals from ecclesiastical courts.

Since 1998 it has also had power to rule on constitutional appeals arising over devolved powers to Scotland and Wales.

In recent years its overseas jurisdiction has reduced as successive countries have cut off the Privy Council as a court of final appeal: Canada, India, Sri Lanka, African nations, Singapore and, most recently, Hong Kong and New Zealand have all withdrawn.

In all it handles about 55 to 65 Commonwealth and devolution appeals a year, appeals nominally to the Queen as head of state. The judges, notes Mr O'Connor, do not make decisions like other courts; they ?humbly advise Her Majesty? whether to grant a petition to the appellant. But the Queen can also refer to it any matter that she wants to. In effect, he says, it is ?an embryonic, but unused, constitutional court?.

The Privy Council, he says, cannot escape any programme of constitutional reform, concluding it is a ?dysfunctional body? with an ?eclectic range of work? - part government, part independent court, part a forum for the monarch?s remaining personal powers and part ?a theatre for benign historical ceremonial?.

From this autumn the Judicial Committee will move to a room in the new supreme court building due to open in Parliament Square. It will be a welcome move of another court in the UK justice system into a more transparent and open world.