Alan Trench: Holyrood and Westminster both face difficult decisions before the vote

DAVID Cameron’s intervention in the independence referendum debate is not much of a surprise. There has been a growing perception among the Unionist parties that they are losing control of events, and increasing annoyance at the way they see the SNP and the First Minister as rigging the terms of the debate against the pro-Union side.

The idea of conferring powers on the Scottish Parliament by an order under section 30 of the Scotland Act 1998 as the means for asserting control of the debate is also not new.

A section 30 order is not simply London laying down terms for a referendum. The First Minister has been very confident in public that Holyrood has powers to call a referendum. While most lawyers would agree that the parliament has powers to call a referendum touching on independence, there are big questions about how extensive those powers are.

Such an order can offer clear powers for Holyrood – the only parliament with a mandate for a referendum – to hold it. Arguments that this is “interfering” with Scottish democracy are unconvincing, as the order would require Holyrood’s assent as well.

From an SNP point of view, this is the moment when they have to make the choice. Do they want to acquire the powers to hold more or less the referendum to which they are committed, even if that’s not quite the referendum they want to hold? Or do they feel confident enough of the parliament’s existing powers to hold one, and reject the order?

It is not plain sailing from the UK point of view, however. The order needs Holyrood’s consent. That means it needs to be framed in terms that are attractive to the SNP to vote for it. Moreover, as it is clear that Scottish voters want much greater self-government within the Union, a referendum that offers a choice between the constitutional status quo and independence is by no means a clear winning proposition.

• Alan Trench is an honorary senior research fellow at the Constitution Unit, University College London.