Some of Britain's leading jewellers have been accused of propping up the military dictatorship in Burma by trading in "blood" rubies sourced from the country.

Asprey, Cartier, Leviev and Harrods are selling the gems in their central London stores, with some items costing as much as £500,000.

Rubies from Burma are among the most sought after in the world and experts claim the military junta makes tens of millions of pounds each year from the lucrative trade.

While America has banned all imports from Burma, the gems continue to be sold in Britain despite government claims that it "discourages" trade between the countries.

Last night Foreign Office sources indicated that Gordon Brown was pressing the European Union to introduce tougher sanctions against Burma which would prohibit sales of its gems in Britain.

"A gift of a ruby is meant to symbolise love, but if it comes from Burma the true price is paid in blood and oppression," said Mark Farmaner, acting director of the Burma Campaign UK pressure group. "Any rubies on sale in the UK will have been purchased at some point from the military and so will be helping to fund that regime."

The decision by some jewellers to sell Burmese rubies contrasts with that of Tiffany & Co, an American company with stores in the UK, which has refused to stock such gems on ethical grounds since 2003.

Last week an undercover reporter from The Sunday Times requested items of jewellery containing Burmese rubies at stores in London.

At the Leviev store in Old Bond Street, she was shown a £500,000 ring boasting a five-carat ruby set in diamonds. "Many collectors want Burmese [rubies]," said the sales assistant. "No one talks about diamonds any more, it's all Burmese or coloured diamonds."

Just up the road at Asprey, a company established in 1781, the reporter was shown a one-carat ruby ring costing £10,000.

On sale at the Cartier store in New Bond Street was a 3.18-carat ruby ring valued at £120,000. A sales assistant later e-mailed details of a 10.04-carat stone costing £1.2m.

"This is currently the largest example of a fine Burmese ruby that we could show you," he wrote. "It is currently part of a major exhibition in the Far East. However, it may be possible to view in the UK in the not-too-distant future."

At Harrods Diamonds, a franchise based in the Knightsbridge department store, rubies from Burma were described as "the best" — and those which are "pigeon blood" in colour the most prized.

More than 90% of the world's rubies come from Burma, but they are often cut and polished in third countries such as Thailand which means they are not classed as being of Burmese origin by customs officials.

Because of this loophole, it is difficult to estimate how much the Burmese ruby market is worth in Britain. Other imports from Burma, including teak wood and clothing, totalled £27m last year, according to customs statistics.

Jewellers in Britain procure their stock from international gemstone dealers who usually buy directly from government-run auctions in Rangoon or from trade fairs in third countries.

Gemstones are Burma's third-biggest export after timber and natural gas and are worth about £145m a year, according to the regime's own figures.

Brian Leber, an American jeweller who campaigns against the trade in Burmese gems, said: "The military regime is receiving a great deal of benefit from the sale of rubies because not only do they control the licensing of all mining operations, but they also have a majority share in every mine in the country and run the auctions."

A spokesman for Asprey said: "We have known the majority of our suppliers for many years. To the very best of our knowledge, these suppliers are not involved in funding conflict and the stones have been manufactured in compliance with internationally recognised 'best practice' principles."

A spokesman for Harrods said products were "sourced by reputable companies adhering to internationally recognised legal and ethical guidelines".

"The stones are purchased and finished by the jewellery brands themselves before being offered for sale."

He added: "There would never be any intentional effort by our sales people to disguise the source of the stones . . . Harrods feels that it is up to the individual to make his or her own buying decisions based on their own philosophy and beliefs."

Cartier denied buying gems directly from Burma: "If we have any Burmese rubies in our pieces then they are vintage." Leviev said it was unable to comment.

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