UPDATE 1-Signet Q4 like-for-like sales down 6.7 pct

(Adds detail, share price)

LONDON, Feb 7 (Reuters) - Signet Group Plc (SIG.L), the world's biggest specialty jewellery retailer, said on Thursday fourth-quarter sales were hit by a tough U.S. market in January and a disappointing Christmas in Britain.

Signet, which last month said full-year profit could be below forecasts, having already warned on earnings in November, said group like-for-like sales fell 6.7 percent in the 13 weeks to Feb. 2.

Group like-for-like sales fell 6.8 percent in the eight weeks to Dec. 29, having shown growth of 3.2 percent in the third quarter to Nov. 3.

"In the fourth quarter, the U.S. business faced a very difficult environment, with January adversely impacted by a change in promotional activity," Chief Executive Terry Burman said in a trading update.

"The UK division had a disappointing Christmas period but saw some improvement in January," Burman added.

Like-for-like sales in the United States, where Signet trades as Kay Jewelers and Jared the Galleria of Jewelry, fell by 8.6 percent in the quarter, while sales in the UK, where Signet trades as H.Samuel and Ernest Jones, fell 1.7 percent.

Group like-for-like sales for the year to Feb. 2 eased by 0.7 percent, with the United States seeing a 1.7 percent drop and the UK a 2 percent increase.

Burman said the UK business achieved "a good performance (for the year), given the very challenging fourth quarter".

Shares in Signet, which have underperformed the UK general retailers' index .FTASX5370 by 18 percent in the past year, were down 1.2 percent at 63 pence at 1248 GMT, valuing the company at around 1.06 billion pounds. (Reporting by Mike Elliott, editing by Will Waterman)