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The Royal Mint have issued a statement defending their issues and pricing, but Richard Lobel, managing director of London-based Coincraft, who has been involved in the coin collecting world since the 1950s, told ATG: "Their prices are often four times what they should be. Many of the limited issues are worth considerably less than the ticket price at the point of sale, with absolutely no hope of ever recouping the cost, let alone seeing values rise beyond that."

Mr Lobel, who has emailed fellow members of the trade to let them know about the boycott, told them: "Let the Royal Mint buy back their own material. I am tired of having to tell collectors that, the silver proof £5 they bought from the Royal Mint at £99.50 is only worth £20 in the trade."

He sees the problem as little less than a scandal. "The Royal Mint is either incompetent in setting prices or cynically exploiting non-expert buyers who think they are getting something special."

However, the Royal Mint told ATG: "The Royal Mint commemorative pricing reflects the significant investment of time, skill and thought undertaken in the origination and production of our unparalleled quality standards. Whilst utilising modern production techniques where possible, The Royal Mint remains faithful to its 1000-year history where there is no substitute for the hand crafting by an expert eye."

Private Company

Mr Lobel also accuses the Royal Mint of getting rid of specialist advisers who know how to value such coins in favour of marketing executives.

"People need to understand that the Royal Mint is a private company seeking profits, not a public body. It is a highly trusted brand because of its name, but it is not living up to that expectation of trust."

To support his argument, Mr Lobel quoted numerous examples of limited issues, including expensive gold coins, which he says are ludicrously overpriced.

Referring to a full page advertisement in a national newspaper leading up to Christmas, in which the Royal Mint offered silver sixpences for £19.95 each with a charge of £2.95 for postage, he said: "Coincraft offer the same coins at £4.95 each. What's more, the Royal Mint advertised them as the 'legendary silver sixpence' struck from 1920-1947 that people used to put in Christmas puddings. But they stopped issuing them in 1946 and it was the silver threepence that went into Christmas puddings, so one wonders whose expertise is being used here."

The Royal Mint is now offering the same coins via their website for £30, having added a presentation case.

Other offers Mr Lobel saw as being ridiculously overpriced included:

• A George III half sovereign priced at £1100 (dealer price £350-500);

• A 2012 Paralympic gold proof £5 (melt value £1205) priced at £2880 (dealer price (£1500-1600);

• A King James Bible £2 gold piece of 2011 issue priced at £995 (dealer price £695).

• A Mary Rose £2 gold piece of 2011 issue priced at £995 (dealer price £695).

"I could go on," Mr Lobel told ATG. "On most non-gold numismatic coins you are going to get less than 50 per cent of what you paid for them."

He said mints in the United States, Australia and other countries were selling comparable coins at half the price set by the Royal Mint.

For example, 2011 proof US silver eagles in their original cases are currently being advertised at $65.95, around £42.25. It has around an ounce of pure silver in it, which is about ten per cent more than the 2011 silver proof £5 being offered by the Royal Mint at £99.50.

Mr Lobel also argues that overcharging has had a clear effect on sales of commemorative issue coins over the past 15 years.

"The 1981 silver proof issues for Prince Charles' wedding to Princess Diana sold 218,000 pieces, but by the time we got to 1997 silver proof issues for the Queen's Golden Wedding saw only 33,000 sell.

"Only 13,379 sold for Prince Charles' 50th birthday in 1998 and only 16,000 commemorating the Queen Mother's death in 2002. For Prince Charles' 60th birthday in 2008, the total was down to 10,000.

"The problem is that the Royal Mint are flooding the market in an attempt to cash in and collectors have got wise to it."

The Royal Mint say that the evidence shows otherwise.

"Recent high demand for iconic products such as the 2012 Sovereign, which sold out in a matter of weeks, is testimony to the fact our pricing is in line with the expectations of the majority of Royal Mint customers," they said in a statement sent to ATG. "In order to maintain this expectation we ensure our product and pricing portfolio is regularly reviewed and adjusted accordingly.

"2012 is a great year for Great Britain and an exciting time for the Royal Mint. We are already experiencing strong demand for many of our commemorative issues supporting the London 2012 Olympic and Paralympic Games and celebrating the Diamond Jubilee. And with products from £2.99 to £100,000 we believe we have something fit for anyone, be they art enthusiast or budding collector."