The latest opinion poll from Scotland shows support for independence down to a new low of 23 per cent. Alex Salmond, Scotland’s charismatic first minister, is very unlikely to achieve the Yes vote he seeks in next year’s referendum. Yet the history of votes on constitutional reform suggests that the very factors that make Mr Salmond’s referendum plan likely to fail make British prime minister David Cameron’s EU referendum plan more likely to succeed.

Australia voted in 1999 on a proposal to make the country a republic. The plan initially enjoyed majority support. For the role of Australian head of state to be a part-time occupation for an elderly lady 10,000 miles away with a demanding day job as Queen of England (and Scotland) is plainly absurd.

But the referendum produced a clear vote to retain the monarchy. The campaign forced advocates of change to spell out what change would mean in practice. The idea of replacing the glamour of royalty with the services of a superannuated Australian politician had little popular appeal. Other attempts to dethrone Her Majesty by popular vote – in tiny St Vincent and microscopic Tuvalu – led to a similar shift in opinion and the same outcome.