Geoffrey Robertson: A manifesto for British republicans

The ancient and rotting legal foundations of this kingdom can be rebuilt by a written constitution

The Royal wedding highlights the absurdity of the constitutional arrangements which require Britain to be reigned over by a White Anglo-German Protestant monarch. The real reason why its religious, racial and sex discrimination remains untouched is through fear that the royalist tapestry would unravel if a single thread were pulled. But if this were ever the case, what could republican ideology – which has scarcely advanced in Britain since the 1650s – offer in replacement?

The bedrock of our constitution is the Act of Settlement of 1701, a blood-curdling anti-Catholic rant, which enshrines the genes and Protestant religious beliefs of Princess Sophia of Hanover in the succession to the throne. This means that any monarch who holds communion with the Church of Rome or who marries a Papist – heaven forbid a Muslim or a Methodist or a Scientologist – is immediately de-throned. The Act imposes anti-meritocratic race discrimination: no-one unrelated to this German family (the Windsors changed their name from Saxe-Coburg Gotha during the First World War to disguise their familial relationship with the Kaiser) can aspire to the crown.

In further breach of the Human Rights Act, this primitive 1701 law adopts the feudal principle of primogeniture (inheritance down the male line), so that if the happy couple produce a daughter, she will be relegated to the bottom of the stately pile, below any subsequent male heirs. If Charles III were to convert to Catholicism (or have a sex-change operation) the crown would go first to his male children then to his male brothers and their family, ahead of their older female sisters.

Tom Paine pointed out the absurdity of a hereditary poet or a hereditary mathematician, although he failed to take into account the entertainment value provided by a hereditary family of royals. Speaking of which, there is still the 1361 Treason Act, which did for Anne Boleyn and will constrain Kate Middleton by criminal law, after Charles becomes King: it punishes (until recently, with death) any party to adultery with (and including) the wife of the heir to the throne.

These constitutional laws are obsolete and obnoxious, and in some cases very silly – for example, ownership of every wild swan is vested in the monarch, and in the case of "the royal fish", the head of every whale, sturgeon or grampus landed in the kingdom belongs to the King, and its tails belong to the Queen. You may not find the monarch's immunity from legal action quite so amusing, however, if you are run over by a royal motorcade hastening to get to the church on time.

Why has reform not seriously been attempted? The Labour government claimed it would be too hard, because the Commonwealth would need to be consulted and countries like Jamaica and Australia were besotted by royal tradition. David Cameron has repeated this nonsense about the Commonwealth wishing to block change. In fact, Commonwealth countries do not need to be consulted: if they still wish to be reigned over by the British royal family they must take it as they find it, after it has moved into the 21st century.

In any case, it is an open secret at the Commonwealth Secretariat that Charles will not be the next head of the Commonwealth when the Queen retires – they are looking for someone more inspiring. Mandela, once the favourite candidate, is now too old, so the crown will probably go to his wife, Graca Machel or, more inspiringly, to Aung San Suu Ki (Burma was once part of the empire). If the Queen holds on for another five years, the role of titular head of the Commonwealth could well be offered to ex-president Obama, who has Kenyan ancestry.

In the meantime, where stands republicanism in Britain? Our A-Level politics course teaches feminism, anarchism, conservatism, multi-culturalism, socialism and ecologism, but republicanism is not on the syllabus. Perhaps unsurprisingly, as there has been little intellectual advance since John Cooke's Monarchy – No Creature of God's Making (1652) and Harrington's Oceania (1657).

Some years ago The Guardian began a legal campaign to challenge the monarchy. It had initial success when the House of Lords judges said that the un-repealed sedition laws which sent Irish republicans to Botany Bay in 1848 could never again be enforced, thanks to the Human Rights Act, but it could not find a Catholic in line to the throne prepared to challenge the discrimination in the Act of Settlement. Although there are many (mainly German) in the first one thousand in line for the throne, all those contacted declined the opportunity to bring a test case, because they feared they would no longer be invited to take tea at Buckingham Palace.

As part of that campaign , there was discussion about how to elect a republican president. This usually ended with Vernon Bogdanor muttering "do you want to have President Tebbit or President Hattersley?" (Now, the spectre would be President Major or President Prescott). But political lobbying to put some spent party hack on the constitutional dais can very simply be avoided, if the 1701 Act of Settlement were replaced by a law requiring a democratic election for Head of State every five or seven years, but excluding from candidature anyone who has held government office.

That would of course permit Prince Charles to stand – he would probably win the first Presidential election unless a royal vote was split by one of his sons (or his sister) standing against him. That might let in Richard Branson or Helen Mirren, or lead to victory for the inevitable "Stephen Fry for Queen" campaign – but why not?

In the Westminster system, the role of head of State, with the duty "to advise and to warn", is an important part of the constitutional mechanism. But Elizabeth II merely follows the directions of the government of the day, however astute she may remain in her old age on picking Derby winners.

Britain lacks an elected and respected figurehead, above party politics, who could provide wise counsel to the government. One result of choosing a Head of State by inheritance rather than election is that in a crisis they may lack the confidence, popularity and independence to make a worthwhile contribution to governance. Good luck apart, the Act of Settlement (1701) and the genes of Princess Sophia will not provide – by sexist, racist or religiously discriminating descent – the calibre of leadership that could be provided by elections every five or seven years for a Head of State.

The challenge for republicans today is to find a way to keep the royal soap opera running, whilst ensuring that public power is exercised by elected representatives or officials appointed on merit. There needs to be a written constitution, in which the Windsor family has a very small part. They would still be called Kings and Queens, would remain custodians of Windsor and Balmoral, and other royal parcels, and they would still hold garden parties and have whisky jam and boot polish "by Royal appointment".

But the monarch would be replaced, for most practical purposes, by an elected President who would represent the nation abroad, advise and warn the government, inspire the people and open and, if necessary, close the parliament. The ancient and rotting legal foundations of this kingdom can be rebuilt, by a written constitution that keeps kings and queens but allots them the minimal and secular role, of keeping harmless traditions alive, in the interests of tourism, the tabloids and David Starkey.

Geoffrey Robertson is a Queen's Counsel, and author of 'The Tyrannicide Brief' (Vintage)