A royal sheikh accused of failing to honour a multi-million pound auction purchase has been compared to an “inveterate gambler” who cannot stop bidding for works of art.

Sheikh Saud Bin Mohammed Al-Thani, who was once described as the world’s largest collector of antiques, is currently locked in a legal battle with three coin dealers, who claim he failed to pay up after successfully bidding over £12 million for a rare collection.

The dealers are demanding the sheikh, who is a cousin of the ruling emir of Qatar, has his assets frozen until he honours his auction debt.

The 45-year-old collector is thought to have invested billions of pounds in antiques and works of art.

But at the High Court in London, Jeffrey Gruder QC, representing the coin dealers, described the sheikh as a “serial defaulter”, who had amassed a pile of unpaid debts to some of the world’s top auction houses.

Likening him to a gambling addict, Mr Gruder said: “He bids, wins and then doesn't pay. One can only conclude that this is a person acting dishonourably and disreputably and with mala fides.

“He is bidding when he knows he's not going to be able to pay. Perhaps in a perverse way he enjoys the process of bidding.

“If one wants to be an amateur psychiatrist, maybe he is like an inveterate gambler who can't stop himself gambling at Ladbrokes every day.”

The case was brought by three coin dealers, A.H. Baldwin and Sons Ltd, Dmitry Markov Coins and Medals and N&M Numismaritcs LLC, who claim they are still owed £12.2 million for coins that were sold at auction in New York.

Described as the “finest assortment of Archaic, Classical and Hellenistic Greek coins ever assembled”, the collection attracted fierce bidding when it went under the hammer.

But the dealers said Sheikh Saud had offered “no explanation or excuse” for failing to pay for the lot once he had won the auction.

Mr Gruder alleged that Sheikh Saud had left a string of unpaid auction debts around the world including £4.3 million to Bonhams and £26 million to Sotheby’s.

However lawyers for the Sheikh vehemently denied the claims.

At an earlier hearing on October 9, the sheikh’s substantial assets were frozen, and the coin dealers have applied for the injunction to continue until the row over the outstanding payment is resolved.

Mr Gruder said: “The fact that he is a member of the Qatar royal family is not relevant because if this is his conduct it doesn't matter who he is.”

At one point the deadlock over payment became so entrenched that the UK ambassador to Qatar - and his opposite number in London - became involved.

But Mr Gruder said: “Despite the best efforts of the two ambassadors, no payment has been forthcoming from the sheikh."

The QC also expressed concern that Sheikh Saud was intending to leave his London base and return permanently to the Middle East.

But Stephen Rubin QC, representing Sheikh Saud, rubbished the suggestion, telling the court: “He hasn't slunk off in the night to Doha," adding that it was "not unusual" for men in the sheikh's position to "go back to the Middle East when the temperatures begin to drop" in the UK.

Mr Rubin told the judge, Mr Justice Haddon-Cave, that his client had been trying to pay for the coins for the past nine months.

He said: “There are no doubt timing issues which go to why he cannot pay at the moment, but that's not a reason to make a freezing order against someone.”

“He is a very well-known and internationally regarded art collector," said Mr Rubin, who denied allegations that he had run up a mountain of unpaid debts.

Mr Rubin also attacked the case against the sheikh as "obviously wrong", pointing out that the dealers had no basis for enforcing a contract of sale with him, as they had merely "organised" the sale of coins owned by an "unidentified" client of A.H. Baldwin and Sons.

The QC added: "The way in which the American complaint is formulated is misconceived; in that it purports to be a claim for enforcement of a 'contract of sale', notwithstanding that the claimants were not on any view parties to any such contract with the sheikh".

There was no evidence before the court on the value of the coins which the sheikh "allegedly agreed to buy", added the barrister, who said that the dealers still have the coins and are free to find a buyer for them if they wish.

Mr Justice Haddon-Cave will issue his judgment at a later date.