Today, the Succession to the Crown Bill will receive its second reading in the House of Commons. If one had to think of one person who would welcome this plan to ‘modernise’ the Monarchy, it would have to be that arch-Blairite, Baroness Jay of Paddington. But she popped up yesterday in her capacity as chair of the Lords’ Constitution Committee, to warn of the potential ‘unintended consequences’ of the Bill and to decry the use of the emergency fast track procedure to rush it through.

Surely some mistake? Actually, no. Baroness Jay arguably knows more than anybody about tinkering with the constitution, having made it her life’s work. Despite the consensus across parties and in the media, we ought to take her comments as a sign there is far more unease about this Bill behind the scenes than there is in public.

A hint of the trouble the Bill could bring, comes in clause 5, which says that once it is passed, none of its measures will actually come into effect until the Lord President, ie Nick Clegg says so. This is because the British Monarch is head of state of 15 other nations too. And although their governments have signed up to the plan, none of them has started the legislative process. So the Act will lie dormant, to be implemented at a later date.

Already, at home, the republicans are enjoying the opportunity to vote on the succession and the Labour MP Paul Flynn has put down a number of mischievous amendments. But the real trouble will come in countries like New Zealand and Australia, where the griping against the monarchy is quite widespread. Will the legislation pass? Or will it be amended to say, for instance, that Prince William should be the last monarch of the Bahamas? And what happens if a government in one of the Realms simply says. ‘Hang on, this Bill seems like trouble, let’s just put the whole thing off.’

The answer is that the Act, having been rushed through in Britain will be on the statute book, waiting for the Lord President to produce his pen to implement each clause in Britain as he sees fit.

This takes us to the essence of the criticism of the Bill. In the name of modernisation it risks introducing two entirely new concepts into the succession to the Crown: doubt, and executive discretion. At the same time, it removes another important convention: pragmatism. For if we were confronted with a brilliant elder daughter, married to a Catholic, we would probably find a way of allowing her to succeed without resorting to a doctrinare process fraught with legal difficulty.

Once you start to think of it like that, you realise that Mr Clegg may be opening yet another can of the constitutional worms he so enjoys feeding to us all, like the AV referendum.

There are numerous unanswered questions which Lady Jay and her committee rightly suggest should be fully debated by Parliament. Supposing, for instance, the Duke and Duchess of Cambridge have a daughter and then a son? Contrary to the spin, the girl will not in law actually be next in line to the throne until the Bill has been passed by all the other 15 Realms. The poor son might find himself as first in line, only to be disinherited in the future. The entire line of succession underneath them would also be rejigged. What would happen if some trouble-makers mounted a legal challenge at any stage?

Then there is the issue of the Church of England. All of my Catholic friends bring up their children as Catholics, believing that is their duty. So, surely, if the Monarch married a Catholic their heir could not be a communicating member of the Church of England and its Supreme Governor, and this could trigger the disestablishment of the Church? Mr Clegg says no, as the Vatican no longer requires Catholic parents to bring up their children in the faith. Really? I am not an expert, but that assertion ought to be challenged.

The Bill will no doubt zoom through the House of Commons today. But when it arrives in the House of Lords, let us hope a proper scrutiny will really begin. When people like the Deputy Prime Minister talk of modernising the constitution, experience suggests we do well to count the spoons.

Tags: Constitutional reform, Nick Clegg, Royal Family, UK politics