COCA-COLA’s biggest brand launch in Britain for decades, the sugar-free fizzy drink Coke Zero, may be in danger of falling flat, according to confidential industry data.

After an initial burst in June and July, aided by hot weather and a huge marketing campaign that aroused the public’s curiosity, sales of Coke Zero appear to have suffered sizeable falls for three consecutive weeks.

Data attributed to AC Nielsen, the market research firm, which have been passed to The Sunday Times, show the volume of Coke Zero’s take-home sales in Britain fell from 2.6m litres a week to 1.8m between late July and the middle of this month. Industry sources also said, without providing supporting data, that Coke Zero was cannibalising sales of Diet Coke.

Coca-Cola declined to comment on the figures this weekend, although sources close to the company insisted that it was “delighted” with the new product’s performance.

Coke Zero was quickly dubbed “Bloke Coke” when executives decided to target young men, a demographic group that usually avoids diet drinks. UK directors said the drink was Coke’s most important new product in Britain for two decades.

The soft-drinks giant’s Atlanta headquarters are particularly keen for Coke Zero to succeed in Britain, a notoriously tough market for the company in recent years. It is also viewed as a crucial benchmark of the way the soft- drinks market is evolving, with consumption of traditional high-sugar carbonated drinks continuing to decline.

It is not only at the hands of consumers that Coca-Cola has suffered. On Wall Street, the once-mighty company has fallen into the shadow of Pepsico, its fierce rival, which has won plaudits for its rapid diversification into healthier soft drinks and water products. Coke, by contrast, has been attacked for its ponderous product development and failure to adapt to fast- changing consumer demands.

Coke’s most recent foray into the British bottled water market ended in disaster two years ago when it withdrew the Dasani brand after a contamination scare. The company was also derided for its marketing of Dasani, which turned out to be processed tap water.

Coke has declined to comment on recent suggestions that it is planning a new push into the water sector in Britain. But people who know the company said it was almost certain to introduce another product here next year.

Coca-Cola’s other brands in Britain include Sprite, Dr Pepper, Powerade and Oasis. Coke has also been linked with takeover bids for successful independent companies such as Innocent Drinks, the smoothie maker.