* 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 the boards of Britain's blue-chip companies but the glass ceiling is yet to crack, with most top jobs still held by men and companies needing to do more to promote women, a government report said on Wednesday.
The third annual progress report found women now occupy a fifth, or 20.7 percent, of positions in FTSE 100 companies, up from 12.5 percent in 2011, within striking distance of a target for women to account for one quarter of board seats by 2015.
But campaigners admitted there was still a long way to go as women only account for 6.9 percent of senior executive roles in the FTSE 100 where there are only four women chief executives.
Two companies in the FTSE 100, commodities trader Glencore Xstrata and miner Antofagasta, still have all-male boards - although they told Reuters they are seeking female directors - and 48 FTSE 250 companies have all-male boards.
Former trade minister Mervyn Davies, leader of the government initiative since 2011, said the figures showed a voluntary drive to boost women around the boardroom table was working and ruled out the need for mandatory quotas such as those introduced in France, Italy, Spain and the Netherlands and planned for Germany.
He said the key to lasting improvement was encouraging companies to drive change within, not legislation or government intervention that critics argue can lead to tokenism and "trophy directors".
"This is about changing the culture of corporate Britain, and using a stick is not the best way to do that," Davies told Reuters after launching the latest report in London.
"Everyone now sees that (having more women on boards) is great business sense so the debate is now moving from the boardroom to executive committees."
He said Britain was leading the way at driving corporate change on a voluntary basis rather than with quotas which are now being mulled in the European Union.
But he said companies whether public or private had to step up to the mark and he has written to all companies in the FTSE 350 urging them to find ways to boost female representation.
"Failure would again raise the unwelcome proposal of compulsory measures. British business credibility is at stake and we need to redouble our efforts," Davies said.
Minister for Women Maria Miller said companies needed to be honest that the culture in Britain was not neutral and it was still "white, male and heterosexual".
The only FTSE 100 female CEOs are Moya Greene at Royal Mail , Carolyn McCall at easyJet, Angela Ahrendts at Burberry and Alison Cooper at Imperial Tobacco.
Capita and Diageo had the highest proportion of women among FTSE 100 boards at 44 percent, followed by Royal Mail, Unilever, and GlaxoSmithKline.
Miller said corporate change was needed to address gender imbalance as well as the pay gap between men and women and rules on executive pay.
"Women don't need special treatment. They just need an equal playing field," she said.
With the spotlight on promoting women, several British banks released targets on gender balance in their annual results for the first time this year.
HSBC aims to have women in 25 percent of senior roles by 2014/15, up from 22.7 percent; Lloyds has a target of 40 percent by 2020 compared to 28 percent now; and Barclays 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" to have an all-male board or executive committee.
"A company that does not get the mood of society on this issue actually deserves to go out of business," he said. (Additional reporting by Steve Slater, Editing by Alison Williams)