By Carla Mozee, MarketWatch

LONDON -- U.K. stocks fell Tuesday, with telecommunications firm BT Group PLC and oil firm BG Group PLC among the shares moving the FTSE 100 index toward its first decline in three sessions.

The U.K. FTSE 100 index gave up 0.5% to 6,841.58, erasing Monday's gain, which left the benchmark at its best closing level since May 14, according to FactSet data.

Marking the sharpest loss intraday was BT , with the shares falling 3% as rival British Sky Broadcasting Group PLC outlined the launch of its first channel dedicated to European football. The Sky Sports 5 channel will show more than 600 live matches next season, and will be at no extra cost to customers with its sports package. Sky also said customers may also sign up for two years of free broadband unlimited service.

But BSkyB shares also lost ground, giving up 1.3%.

A downgrade to a neutral rating from outperform by Exane BNP Paribas triggered a 1.4% decline for oil company BG . "Expectations around corporate action at BG appear high to us, and we believe there is no room for half-baked measures for BG's potential new CEO," wrote analyst Alejandro Demichelis in a note Tuesday. "While we see scope to unlock value, we feel the market is underestimating the task ahead."

BG in late April said Chris Finlayson resigned as chief executive for personal reasons, and that its 2014 production will be at the lower end of guidance.

At the same time, Sports Direct International PLC shares sagged 1.5% upon removal from Goldman Sachs's conviction-buy list, "as the upside to our target price is now lower following a period of good performance. We have higher conviction elsewhere in the European retail sector as a result," said analysts at Goldman.

But appearing in the plus column were shares of Imperial Tobacco Group as they gained 0.4%. The firm said it plans to spin off its Companía de Distribucion Integral Logista Holdings S.A. unit in an initial public offering in Spain.

Further signs of strength in the British economy arrived Tuesday as the Office of National Statistics said U.K. manufacturing output rose by 0.4% on the month in April, and by 4.4% on the year.

April marked the fifth consecutive rise for manufacturing output, and the U.K. economy "is increasingly firing on all cylinders, setting up the second quarter for another strong growth reading," wrote Rob Wood, chief U.K. economist at Berenberg, in a Tuesday note. The data support its forecast for an acceleration of second-quarter growth to 0.9% on a quarter-over-quarter basis, "and for the first [Bank of England] rate hike to come in November 2014."

More news from MarketWatch.com:

Boston Fed president lays out cautious exit plan

Chinese natural-gas demand to nearly double: IEA

China allows 10 IPOs after months of pause

Subscribe to WSJ: http://online.wsj.com?mod=djnwires