When Gordon Brown moved into No 10 and was deciding which parts of the constitution needed reforming most urgently, he opted against tackling the bit that bars Roman Catholics taking the throne. Although, in common with a sizeable consensus (from Tony Blair to Jeffrey Archer), he thinks the 300-year-old Act of Settlement is indefensible, it was not his main priority.

Under the 1701 law, Catholics cannot become king or queen and the monarch is not allowed to marry a Catholic. It was designed to settle the succession to the throne on the heirs of the Electress Sophia of Hanover, a granddaughter of James I of England (VI of Scotland), and so continue the Protestant line.

Brown probably figured that aside from the occasional mini-crisis whenever a minor royal marries a Catholic, the act did not feature large in voters’ consciousness. He possibly thought it could wait for another day, or another prime minister even.

Big mistake. Brown, in failing to include the repeal of the act in his constitutional reform programme, has handed a golden opportunity to Alex Salmond, leader of the Scottish Nationalist party, first minister of Scotland — and opportunist par excellence.

On Tuesday Salmond will announce his long-awaited (in Scotland anyway) white paper on an independence referendum and in it will be a section calling for an end to the Act of Settlement.

Brown should have seen this coming — Salmond had been lobbying Blair for 10 years to revoke the act. But perhaps the new prime minister believed that as this constitutional matter was reserved to Westminster and therefore outside the remit of the Scottish parliament, it would go away.

It won’t — and nor will Salmond. In his first 100 days in office the SNP leader has made obvious his determination to challenge London in as many ways as he can. Despite a belated outbreak of warmth in his relations with Brown, he has carved a place carping from the sidelines of British politics, with the potential to do more damage than the opposition in Westminster.

If David Cameron cannot land a blow on the prime minister, the Scottish Nationalist leader will happily do it for him.

Before Salmond was elected he drew up a confrontation master plan and he has more or less stuck to it, digging in over North Sea oil revenues, the replacement of Trident and, most recently, demanding a regulatory role over broadcasting. He has also told Brown to restart the joint ministerial committees, set up in 1999 but rarely used, to promote cooperation between all the devolved administrations.

In Scotland it is easy to forget sometimes that the SNP runs a minority administration and that Salmond governs by the skin of his teeth. Even he admits that he hasn’t the arithmetic in the parliament in Edinburgh to win approval for his independence referendum, but his hope is that in opening the discussion he will, by strength of argument, convert the masses.

Already he is succeeding incrementally, convincing sceptics issue by issue that he is in Scotland’s best interests, even if nationalism is not, and scoring high points in the popularity stakes. A poll on Friday suggested that if an election were held tomorrow the SNP would win easily.

Most MSPs now back his call for more powers for the Holyrood parliament and he will have no trouble persuading people that the Act of Settlement is a breach of human rights. Championing a repeal of the act suits his purposes on a number of fronts. Getting rid of it would not only be a populist move but would also please the significant proportion of Scotland’s electorate who are Catholics and traditionally Labour supporters.

Two months ago Salmond forged a pact with Cardinal Keith O’Brien, head of the Roman Catholic Church in Scotland, to campaign against the Act of Settlement. They vowed to follow their pact with “initiatives to raise awareness”, starting it seems with Tuesday’s white paper.

In entering such emotive territory, Salmond drives another wedge between himself and Brown. The latter, although a son of the manse, is not prepared to stick his neck out over a law that picks on Britain’s 4m Catholic voters, but which does not pick on Hindus, Muslims or those from other faiths who are free to marry a monarch.

What does it matter if Salmond is powerless to change the British constitution and can’t even command a majority in his own parliament? His grandstanding has the desired effect of presenting him in a favourable light — and not just on this. Voters in England glance north with envy over the perceived gifts that Salmond bestows on Scots, from free care for the elderly, to free university education, to free prescriptions and a freeze on council tax.

The fact that he could not pay for any of this without Westminster largesse (Scotland receives an extra 20% per head in public expenditure than England) does not detract from the image of a beneficent first minister, who is everything to everybody. A racing man, he has a canny knack of backing the right causes.

However, the Act of Settlement is not one of them and Brown has solid reasons for not pursuing this constitutional rollercoaster. Revoking the act could lead to the disestablishment of the Church of England: one of its provisions is that the monarch must swear to defend the faith and be a member of the Anglican communion, but a Catholic monarch would, like all Catholics, owe allegiance to the Pope.

Even Blair, who thought the act “plainly wrong”, steered clear of it, citing legislative difficulties. Ministers have insisted the law can be altered if and when an heir to the throne wishes to marry a Catholic and that there is no antipapist prejudice at the heart of British government. Brown should do his best to ignore the irritant north of the border and concentrate on more necessary reforms.

Top of his list should be another settlement — devolution, one of his predecessor’s landmarks and an example of what happens when you start tinkering with the constitution. If the Scotland Act had been better conceived we would not have the iniquity of the West Lothian question. And if the Scotland Act hadn’t been passed at all, Brown would not have Salmond.

Minette Marrin is away

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