A duet between late U.S. vocalist Eva Cassidy and U.K. singer-songwriter Katie Melua has become the first single to top the British charts on sales through a solitary retailer.

The duet topped the Official U.K. Chart Co. listing when it was unveiled last night (Dec. 16). It grafts a vocal by Cassidy from her 1996 album "Live at Blues Alley" (Blix Street) on to a new recording by Melua of the much-covered ballad, which was originally an international hit for Louis Armstrong in 1968. It was also the final song performed live by Cassidy, on Sept. 19, 1996 at the Bayou club in Georgetown, DC. Cassidy died Nov. 2, 1996 after a lengthy battle with cancer.

The new version, released Dec. 10 in the United Kingdom on Dramtico Entertainment and distributed by Pinnacle, was produced by Dramatico chairman--also Melua's songwriting partner/producer--Mike Batt; Cassidy's original recording was helmed by U.S. producer Chris Biondo. The single is now a strong contender to take the much-coveted U.K. Christmas No. 1 slot.

According to the OCC, first-week sales of the single were 55,600, with 97% of those being on CD.

All profits from the sales of the single are going to the British Red Cross and will be spent on the charity's services within the United Kingdom. Mass merchant Tesco accounted for 4.3% of U.K. record buyers' singles expenditure in 2006, according to labels body the BPI. The British Red Cross is Tesco's nominated charity for 2007.

Dramatico struck an exclusive deal with Tesco, which, with just under 2,000 stores, is the largest retailer of any kind in the United Kingdom. Its sales totalled £46.6 billion ($94.1bn) in the year ended Feb. 24, 2007. The physical single is only available in Tesco stores and online at www.tesco.com. At the moment, it is only available in the United Kingdom. Overseas plans are not yet finalized.

In a statement, Melua says: "When we first thought of releasing the single, we did not have a chart position in mind, our aim was purely to raise as much money as possible for the British Red Cross."

Its No. 1 status, she added, "means that the people of Britain have once again shown how generous they are when it comes to helping other people."

Tesco corporate responsibility director Ruth Girardet added: "'What A Wonderful World' has had a fantastic week of sales, with thousands ordering and downloading the single from Tesco.com or buying it in store." She commented: "This is a perfect way to end a successful year of fundraising for the British Red Cross."