“Give the people a vote” is the easiest, and most popular, call for any politician to make. But when? Referendums have become relatively frequent in the UK over the past 35 years. Nine have been held, though only one has been nationwide, and several more have been promised. But there is no agreement about when they should be called. This is not just about the European reform treaty, but goes much wider to the balance of parliamentary and direct participatory democracy.

It all goes back to the absence of a written constitution and rule by precedent, or rather political expedient. Many European countries have written constitutions that lay down when referendums can and should be held. In most cases, this is when there is a total, or substantial, revision of that constitution. Ireland and Switzerland (which is outside the EU) are the most definite, and, in the former case, this covers EU treaties.

In other countries, there is greater latitude. Calling a referendum is up to the government, or a certain number, or percentage, of the legislature or the electorate.

In Britain, however, each referendum is authorised by a separate Act of Parliament, though an enabling measure has permitted local ballots on mayors. The most contentious ones, virtually all regarding Europe, have been proposed as a way out of divisions in the ruling party. But it is no longer entirely ad hoc. There is now a certain amount of case law. Referendums are held when new representative bodies are proposed. Even though the Scottish Parliament and the Welsh Assembly were established by statute, it would be impossible in practice to abolish them, or substantially change their role, without a new referendum.

In addition, ministers have at various times promised referendums on membership of the eurozone and on changes to the voting system for the Commons (both in abeyance), and at least floated the idea of one on the future of the Lords.

It is now understood that when a fundamental constitutional change is proposed, it should be subject to a referendum. Gordon Brown has made this point in distinguishing between the old constitution and the new reform treaty.

The trouble is that constitutional significance lies in the eye of the beholder. Many Tories who voted against a referendum on the Maastricht treaty in 1992-93 now back one, even though the latest proposals are, arguably, less significant than Maastricht. That is leaving aside the essentially political, rather than constitutional, argument about whether Mr Brown has broken Labour’s 2005 manifesto commitment by not holding a referendum. William Hague has gone farther in saying that all changes to the running of the EU should be put to a referendum.

But who should decide when voters rather than Parliament should have the final say? A group of “wise men”, the new Supreme Court (as the law lords will be called in a couple of years), or should there be a threshold for a demand by MPs and/or voters? And in the world of e-democracy, it is now possible for voters to express opinions directly, as more than five million have over the past year on the 10 Downing Street petitions website. Sorting out the how and when of referendums should be a priority for the promised constitutional reform debate.

Times Recommends

• Obama is giving his party whiplash
• Mamma Mia, how can we resist them?
• This loyal soldier deserves promotion