Is it easier to forge a £1 coin or a Leonardo?

'In theory, any technological efforts to prevent forgery can potentially be overcome,' he says. 'It's all very well to have technological means to detect forgeries – it’s very much another matter to detect them in all the billions of transactions.'

The Royal Mint has released a new 12-sided ‘counterfeit-proof’ £1 coin after an influx of fake round £1 coins into the UK.

Professor Kemp is not a coin expert, but when it comes to forgery he knows what he is talking about – he is perhaps the world expert in detecting forged Leonardo da Vinci paintings, having spent 50 years in what he calls the 'Leo business'.

'I get sent lots of dodgy Leonardos,' he says. 'One was a very clever forgery of a mechanical which was going to be sold in one of the major London auction houses.'

What happened after Professor Kemp unmasked the painting as a forgery? 'It's now gone underground,' he says mysteriously.

Professor Kemp says there is more of an incentive for conmen to forge money rather than art. 'The potential number of forged coins is huge and the returns massive,’ he says. ‘With works of art, the question is whether it is worth the effort, given the difficulty, time, skill and costs,' he says.

Nor is all art equally difficult to forge. Modern art is much easier than the Old Masters. 'Older paintings were elaborately-made, layered structures using historic materials - very different from modern ones,' he explains.

'The old materials can be replicated to a degree but it is hard to get round all the modern methods of scientific examination. It’s easier to fake a more recent painting - say a Russian abstract work in oil paint from 1914.'

Arts Blog accepts no responsibility for any readers who now try to flog a Russian abstract painting at Christie’s.