Top US jewellers are facing new competition from a surprise source, the wholesale club operator Costco.

The likes of Tiffany and Harry Winston will now have to find ways to hold on to consumers as the company moves into more upmarket jewellery.

A spokeswoman explained to CNN that although the store has always sold jewellery, a new ring at the company "is the most expensive piece" they have ever had for sale.

At present a yellow diamond ring is up for sale on the firm's website for $180,000, following the sale of a $98,000 1.44 carat pink diamond ring.

The new item features a 10.61 carat centre stone, set in a platinum and yellow gold band, with yellow and white pave diamonds.

Fully certified by the International Gemological Institute (IGI) and holding a total carat weight of 12.19, the ring is well priced since it is actually valued at $264,765.

The firm is known for selling large amounts of goods at reduced prices and appears to be taking this approach with its luxury jewellery too, meaning other jewellers could be up against considerable competition.

Costco says it will develop both platinum and gold pieces, and could be set to make a big impact on the jewellery retail market.