UPDATE 1-Women struggling to crack glass ceiling in top UK companies-report

* Women now hold fifth of FTSE 100 board positions

* Few women on executive committees with only 4 CEOs

* Campaigners push for voluntary change, not quotas (Adds new quotes, numbers, after report launch)

By Belinda Goldsmith

LONDON, March 26 (Reuters) - More women are sitting on theboards of Britain's blue-chip companies but the glass ceilingis yet to crack, with most top jobs still held by men andcompanies needing to do more to promote women, a governmentreport said on Wednesday.

The third annual progress report found women now occupy afifth, or 20.7 percent, of positions in FTSE 100 companies, upfrom 12.5 percent in 2011, within striking distance of a targetfor women to account for one quarter of board seats by 2015.

But campaigners admitted there was still a long way to go aswomen only account for 6.9 percent of senior executive roles inthe FTSE 100 where there are only four women chief executives.

Two companies in the FTSE 100, commodities trader GlencoreXstrata and miner Antofagasta, still haveall-male boards - although they told Reuters they are seekingfemale directors - and 48 FTSE 250 companies have all-maleboards.

Former trade minister Mervyn Davies, leader of thegovernment initiative since 2011, said the figures showed avoluntary drive to boost women around the boardroom table wasworking and ruled out the need for mandatory quotas such asthose introduced in France, Italy, Spain and the Netherlands andplanned for Germany.

He said the key to lasting improvement was encouragingcompanies to drive change within, not legislation or governmentintervention that critics argue can lead to tokenism and "trophydirectors".

"This is about changing the culture of corporate Britain,and using a stick is not the best way to do that," Davies toldReuters after launching the latest report in London.

"Everyone now sees that (having more women on boards) isgreat business sense so the debate is now moving from theboardroom to executive committees."

BUSINESS CREDIBILITY

He said Britain was leading the way at driving corporatechange on a voluntary basis rather than with quotas which arenow being mulled in the European Union.

But he said companies whether public or private had to stepup to the mark and he has written to all companies in the FTSE350 urging them to find ways to boost female representation.

"Failure would again raise the unwelcome proposal ofcompulsory measures. British business credibility is at stakeand we need to redouble our efforts," Davies said.

Minister for Women Maria Miller said companies needed to behonest that the culture in Britain was not neutral and it wasstill "white, male and heterosexual".

The only FTSE 100 female CEOs are Moya Greene at Royal Mail, Carolyn McCall at easyJet, Angela Ahrendts atBurberry and Alison Cooper at Imperial Tobacco.

Capita and Diageo had the highest proportionof women among FTSE 100 boards at 44 percent, followed by RoyalMail, Unilever, and GlaxoSmithKline.

Miller said corporate change was needed to address genderimbalance as well as the pay gap between men and women and ruleson executive pay.

"Women don't need special treatment. They just need an equalplaying field," she said.

With the spotlight on promoting women, several British banksreleased targets on gender balance in their annual results forthe first time this year.

HSBC aims to have women in 25 percent of seniorroles by 2014/15, up from 22.7 percent; Lloyds has atarget of 40 percent by 2020 compared to 28 percent now; andBarclays is eyeing 26 percent by 2018, up 5 points.

Davies, the former chief executive of Standard Chartered, said it was "inconceivable in this day and age" tohave an all-male board or executive committee.

"A company that does not get the mood of society on thisissue actually deserves to go out of business," he said. (Additional reporting by Steve Slater, Editing by AlisonWilliams)