By Carla Mozee, MarketWatch

LONDON (MarketWatch) -- U.K. stocks dipped Friday, with shares of insurance firms selling off on plans of an investigation by the U.K. Financial Conduct Authority into older pensions and savings plans.

Insurers were the worst-performing stocks, slumping following reports of the probe, which is investigating whether consumers who were sold pensions and savings plans before 2000 are being treated unfairly compared with newer clients, including facing costlier fees.

The U.K.'s FTSE 100 lost grip of earlier gains and slipped nearly 3 points to 6,586.31. The index has been up by as much as 0.7% during the session.

On Friday, shares of Resolution Ltd. plunged 15%, Aviva PLC slid 7.8%, Legal & General Group PLC lost 7.1% and Standard Life PLC fell 4.3%. Also, Prudential PLC gave up 4.9%.

Insurance stocks saw heavy losses recently after U.K finance minister George Osborne, as part of his annual budget, announced plans to end a rule that required pension funds to be used to buy annuities, a key portion of business for some insurance companies.

Commodity producers, meanwhile, held to higher ground, with energy issues up after oil futures (CLK4) extended their rise above $101-a-barrel level. BP PLC (BP) tacked on 0.8% and Royal Dutch Shell PLC rose 0.6%.

On the economic front, the Office for National Statistics said gross domestic product in the U.K. in 2013 expanded by 1.7%, lower than a previous estimate of 1.8%. The growth rate of 0.7% for the fourth quarter was unrevised.

More news from MarketWatch:

European stocks gain after consumer-confidence data

BlackBerry Ltd. posts $423 million loss

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