Responding to mounting concerns that big shareholders can exert undue influence on firms with equity that is thinly traded on London's blue-chip FTSE 100 index, owner FTSE Group is lifting its minimum free-float rule to at least 25%, the company said Wednesday.

FTSE, which operates indexes including the FTSE 100, said in a statement that, in a market consultation, 83% of respondents said the FTSE should apply a minimum free-float threshold of 25% for U.K.-incorporated companies in FTSE indexes. The previous threshold was 15%, with exceptions for some very large companies.

The move is a response to growing concerns among institutional investors that companies--often with most of their operations outside the U.K.--reap the benefits of increased liquidity and access to cheap debt and equity-issuance that a FTSE-inclusion brings, without the need to abide by the corporate governance standards that U.K. institutional investors require.

A number of oligarch-controlled Russian companies have listed in the U.K. in recent months, including Polymetal International PLC (POLY.LN) and Evraz, which recently gained entry into the prestigious FTSE 100 by swapping Moscow-listed shares and global depository receipts for shares in new holding companies. Meanwhile, Glencore International PLC (GLEN.LN), the commodity trading giant, was included in the FTSE 100 days after listing with about a 16% free float.

"With the guidance and direction of FTSE's independent practitioner committees, FTSE regularly consults the market on changes to the ground rules of its indices to ensure that the indices continue to meet investors' requirements and define and lead global standards in indexing," FTSE Group's Mark Makepeace said in a statement. "Both FTSE group and our shareholder, the London Stock Exchange Group, recognize that this strong stance on governance underpins the value of our business."

The Association of British Insurers, which had called on FTSE Group to raise the threshold, welcomed the move, which a spokesman described as "a welcome step towards addressing our concerns."

The FTSE policy change will give companies whose free floats now fall below the 25% threshold two years either to raise their free float or lose their index status. It applies only to U.K.-incorporated companies included in the FTSE All-Share index, which includes the FTSE 100, the FTSE 250 index and the FTSE 50 index.

The rule will come into force for new entrants Jan. 1, 2012.

Six companies, largely concentrated in the resources sector, will be affected by the policy changes are Essar Energy PLC (ESSR.LN), Eurasian Natural Resources Corp PLC (ENRC.LN), Evraz Group PLC (EVRZ.LN), Fresnillo PLC (FRES.LN), Glencore and Ferrexpo PLC (FXPO.LN), people familiar with the matter said. All are U.K.-listed resource companies in the FTSE 100 index and all except Glencore are domiciled in the U.K.

Some of the companies indicated that they were prepared to take steps to bring their free floats in line with FTSE Group's threshold as soon as possible. ENRC, in a statement, said it would take "appropriate steps" to comply. ENRC's free float is 18.4%. The chief executive of Russian steelmaker Evraz, Alexander Frolov, said it would be easy to place a small stake in the market to allow the company to comply with th new listings rules. Evraz currently has a 24.8% free float.

ENRC this year became a particular focus for worries about corporate governance of some U.K.-listed firms after it ousted two independent board members.

-By Jessica Hodgson, Dow Jones Newswires; +44 (0) 7561 424788; jessica.hodgson@dowjones.com

(Rhiannon Hoyle contributed to this article.)