Imagine being convinced you own an old master, but repeatedly being told it's a fake.

That is what art collector David Joel says has happened to him and he has spent nearly two decades trying to get his Monet accepted as genuine.

It looks authentic and carries Claude Monet's signature on the bottom right hand corner.

But Mr Joel could never put it up for auction because officially it is a fake - but he is convinced otherwise.

Now aged 82, he can still recall the day he first saw the painting and fell in love with it - although initially it was well beyond his price range.

He said: "I first saw it in a sale room in Norwich. I couldn't possibly afford it because it was supposed to go for £500,000. I really loved it but there was nothing I could do about it.

"Two years later I heard there was a possibility I could buy it from the owner and I bought it for £40,000."

Missing Monet

Mr Joel believed the painting Bords de la Seine a Argenteuil was a missing Monet. It shows a scene identified as close to the painter's home on the Seine outside Paris. A note on the frame suggests it was painted in 1875.

How to spot a fake Shine a halogen torch at the painting to check the brush strokes - the unique handwriting of the artist.

Date the wood used in the frame. Dendrochronology - the date analysis of tree rings can be used for this.

Smell it. Oil paintings will have an oily smell for many years until the oil fully dries. An old painting shouldn't have that smell.

However his exhaustive attempts to get accredited have so far failed.

The final word on whether it is genuine rests with a handful of individuals in a billionaire family of art collectors, dealers and art scholars - the Wildensteins.

The Wildensteins have, since the early 20th Century, been considered one of the world's leading authorities on painting, particularly the work of the Impressionists.

Without their approval, any scientific claim about the validity of the painting is worthless.

They adjudicate on all claims over Monet paintings. Their decision is final and there is no right of appeal.

Mr Joel has exchanged dozens of letters with the Wildensteins but without success.

"I have been trying for 18 years now to get Wildenstein to accept it for his catalogue. It is a long haul but I shall win in the end I think."

Art dealer and Antiques Roadshow expert Philip Mould said if the painting could be authenticated it would be worth a great deal more than £40,000.

"If it were included in the book it would be a very different picture. People would look at it differently, people would value it differently. It could be worth over £1m."

Long process

Mr Mould believes there are many other pictures out there with a greatness yet to be fully realised.

Find out more Fiona Bruce and Philip Mould will examine the evidence for the missing Monet. Can they prove its authenticity?

Fake or Fortune will be on BBC One at 1900 BST on Sunday 19 June 2011. Catch up via iPlayer Read more about Fake or Fortune

But for anyone who has such a painting, proving its authenticity can be a long process.

You need to have history, science and a few art experts on your side.

The best proof is to have the documents showing an unbroken link between the painting's owners. This has been made easier in recent years by being able to access wills online going back to the 18th Century.

Inscriptions and labels on the back can provide valuable clues. Since the early 19th century Christie's have put a numbered auction stamp on the back of every painting enabling people to refer back to its records to see who bought and sold the painting.

Science now also has a much greater role in establishing authorship.

"It has advanced massively," said Philip Mould. It can establish all sorts of things which before couldn't be done. For example you can look into a picture and see how the various layers have evolved.

"In the past you can say we looked at pictures, now you can say we look through them.

"But however much science and history you are able to bring to bear, ultimately a scholarly eye is critical, particularly for old masters."

Who you present your evidence to will depend on the painting you have.

"It is an evolutionary process that has taken place in the arts world with all sorts of different outcomes," said Mr Mould.

In France it was often the descendant of the artist that was given the moral authority to make a final decision on whether a painting was genuine.

But in the Netherlands, when it came to Rembrandt paintings, a committee of six experts would make the decision. There was also a Rubens project and a Van Gogh committee.

There could never be a "one size fits all" when it came to final arbitration but there were some "disturbing anomalies".

But for anyone like Mr Joel who believed he owned a masterpiece it was important to know for certain and have that ownership recognised.

"It's that faith, the knowledge that it was touched by the man or woman in question," he said.

It was the human aspect that gave the value to art. With that a painting could be worth £1.5 million rather than £1,000 in just the same way that a dress worn by Princess Diana might fetch £100,000 against £1,000 for the same dress not worn by her.

Fake or Fortune is presented by Philip Mould and Fiona Bruce. The first programme Monet can be seen on BBC One on Sunday 19 June at 1900 BST and after on BBC iPlayer (UK only)