Please suspend your current beliefs, let your imagination run free and come with me on a white-knuckle constitutional funfair ride. Here goes.

In five years' time, the Queen abdicates and Prince Charles renounces the throne. Prince William becomes king. He also announces his intention to wed, adding that the identity of his fiancee will be revealed at a press conference. There are gasps from the assembled media as a curtain is pulled back to show the very voluble, very Catholic Ann Widdecombe. The tabloids have sport with the notion of a marriage between King Billy and a high-profile Roman Catholic. The king then reveals that he will convert to Catholicism - and, furthermore, that he will refuse to sign into law any bills that run counter to the view from the Vatican. The traditional Protestant nightmare is complete.

This kind of scenario - albeit a less absurd version of it - could come to pass if Gordon Brown follows through on his commitment to consider abolishing the Act of Settlement, which bans Roman Catholics from the throne and even prevents the monarch from marrying a Catholic. I hope the 1701 Act is chucked into history's bin. This piece of legalised anti-Catholicism, which was originally designed to allay legitimate Protestant fears of rule by a "divine right" dictatorial Catholic monarch with a hotline to Rome - all at a time of ferocious religious persecution - should have no place on Britain's statute books in the 21st century.

The main arguments made against repeal are that there are much more important issues claiming parliamentary time, and that the establishment of the Church of England would be at risk. I can't see how the legal incorporation of a sectarian state can be justified under any circumstances; the time for grasping this particularly unpleasant nettle is long overdue.

So what about the threat to the Church of England if the monarch was a Catholic? It's time this one was sorted as well. It's a nonsense that someone should be "supreme governor" of the C of E simply because of who his parents are. What if the eldest son was an atheist? (It would be even more fun if Prince William announced that he was gay and wanted to become a priest.) And is it right for the Archbishop of Canterbury to be chosen by the prime minister of the day? Nah. Complete madness. And, while we're at it, why should the House of Commons be able to vote on the Church of England Prayer Book? Is the Beast of Bolsover a liturgical expert?

The notion of ecclesiastical establishment goes all the way back to the Holy Roman Empire. Catholic Christianity became the official state-approved religious ideology. The Pope crowned the emperor, who in turn protected the pontiff. The arrangement did provide a certain amount of stability, but it's long, long past its sell-by date.

The establishment of the C of E is convenient in some regards; it solves the problem of who will preside at royal marriages. (Mind you, ecclesiastical knickers were in a serious twist and Rowan Williams's eyebrows were in overdrive over the marriage of Charles and Camilla.) But there are two main problems with church establishment. The first is that the privileged church too easily become a toothless dog - a corgi? - and the second is that, when a once-strong allegiance to the state church becomes a minority sport, that particular game is largely over. The same applies to the Church of Scotland.

The biggest elephant in the constitutional living room, though, is the monarchy itself. Monarchy is based on privilege for the few through heredity. It stands at the pinnacle of the class system of this country. It's ridiculous in today's world that we bow and curtsy to people just because they were born to particular parents. I know all the arguments about tourism and heritage, but isn't it time we left this absurd, expensive, Gilbert and Sullivan stuff behind? I think the Queen is an admirable human being, but the whole system reeks of deferential nonsense.

But if we give up the monarchy, won't we have to replace the royals with failed politicians? Not unless we choose to do so. We are free people. Ireland has dealt with this splendidly, choosing interesting people as presidents. If, for instance, Scotland were an independent country, who might be its ambassadors? Let me throw out some names off the top of my head: Liz Lochhead, John Miller, Kathy Galloway, Don Paterson, Alison Elliot, Norman Drummond, Annie Lennox, Tom Fleming, Catherine Lockerbie, Richard Holloway. There's stacks more out there. It's time to move away from deference to privileged royals and petted-lip prelates.

Go for it, Gordon. Let fresh air gust through our creaking constitutional arrangements, with a hey and a ho and a hey-nonny-no. Let freedom roll.