Women may have been battling the glass ceiling for 30 years, but for the lesbian, gay, bisexual and transgender communities – LGBT for short – equally pressing is the "pink plateau" that in many organisations still sidelines non-heterosexuals and denies them the pick of top jobs.

While many blue-chip employers are making efforts to puncture homophobic attitudes with robust diversity policies – partly for reasons of legal compliance, but also as a reaction to the talent shortage – in reality, day-to-day discrimination may still be as pernicious as ever.

That's the view of the gay marketing specialist Out Now Consulting, which believes that around half of all lesbian or gay workers in the UK may be hiding their sexuality from colleagues.

Out Now's 2008 Gay Market Study, carried out among 1,231 readers of GT, Pink Paper and Diva, suggests that despite the UK boardroom's outward willingness to embrace diversity, 82 per cent of lesbians and 75 per cent of gay men believe that coming out at work could harm their promotion prospects.

Some 13 per cent of lesbian respondents and 14 per cent of male respondents said they had, in the last 12 months, personally experienced harassment at work related to their sexuality and only 25 per cent of male respondents and 18 per cent of female respondents thought that being out at work would have "no effect" on their long-term careers.

"There have definitely been significant advancements made at legal and policy levels and I have no wish to belittle them," says Ian Johnson, Out Now's founder and principal consultant, "but our latest research shows that while these policy changes are admirable, they are not making it down to the shop floor.

"There is now a significant disconnect between improved policy positions on workplace equality and diversity and the everyday realities of life for Britain's many gay workers and it is clear there is still a lot of ground to cover before the UK can claim a true sexuality-neutral and equal opportunity work environment."

Leading gay rights lobbying group Stonewall takes a far more optimistic view. "The pink plateau does still exist and it's similar to the glass ceiling in many respects, even though, unlike gender, people's sexuality is invisible," says David Shields, director of workplace programmes.

"Nobody can tell you with any accuracy how many gay people there are in senior positions in the UK, but our latest Workplace Equality Index suggests that around 60 per cent of the organisations who work with us have 'out' and visible role models at chief executive or managing director level and that figure is up from 42 per cent last year."

Although Shields recognises it is likely that many more people in top jobs have so far chosen not to be open about their sexuality – "coming out has traditionally meant long-term damage to your career prospects," he says – why should any employee's choice of sexual partner be of interest to their firm?

Shields argues that being open about your identity at work can make you more productive, happier and less likely to quit.

"We believe that if firms want to get the best out of their people, they need to create a culture where they feel free to be themselves. For the senior people, this will inevitably lead to more authentic leadership and for the workforce generally, it is clear that putting your energy into hiding your sexual orientation leaves less energy for doing your job well," he says.

Shields adds that while at first glance it may appear unimportant, the creation of a more open business culture can even come down to the language that a firm chooses to adopt.

"When a company specifically invites 'husbands' or 'wives' to an event, rather than 'partners,' gay people can feel very unwelcome. We welcome the fact that many more employers are now choosing to ignore the heterosexual norms in the language they use to staff."

IBM agrees that diversity policies are as much about boosting productivity and retaining talent as they are about staying on the right side of the law.

"We have had a general employee diversity policy in place since 1953 and although we are not able to collect specific statistics on the sexuality of our staff, we believe that IBM senior management not only understands the issues but is geared up to fully support any member of staff who does decide to come out," says Scott Stockwell, diversity group leader at IBM UK.

"We work hard to create an environment which gives people the choice to be open about their sexuality and which allows them to give their best at work – without having to pretend that they are someone different. We promote solely on merit in this organisation and the fact that some of are senior people are both 'out' and highly visible speaks for itself."

It has become clear over the past three decades that although they are still few in number, women business leaders invariably contribute something new and different to their organisations.

Is the same true of LGBT high flyers? Yes, says David Shields: "If you've faced discrimination, you inevitably build up a more diverse perspective on life and hopefully a more open form of leadership. I believe that many gay business leaders tend to understand their customers, staff and the business generally far better."

'In the last 15 years the legal profession has really moved on'

Adrian Barlow, 45, is head of the property group at international law firm Pinsent Masons, which he joined as a trainee in 1985. He manages 180 lawyers, 53 support staff and a turnover of £35m a year.

While I was still fairly new to the firm, I made an attempt to come out one evening when I was very drunk, but I took the advice of a member of my team who told me to keep quiet. Although I was seen as a high flyer, having to pretend made me feel very disenfranchised and depressed.

I went to see a law firm that was advertising itself as exclusively gay at the time, but soon realised that although we had our sexuality in common, we didn't share much else.

After a conversation with the then head of Stonewall, I decided to stay at Pinsent and try and change the place, rather than place myself in a ghetto.

On the eve of being offered a partnership, I went to see my divisional head and told him that I was gay and that if I wasn't allowed to be myself, I'd rather resign.

He told me that it was fine as long as I didn't embarrass the firm – which was a real cheek, looking back on it – but in the last 15 years the world and the legal profession have really moved on and that sort of comment wouldn't be made today.

My sexuality has never come into any discussion I've had with clients and when I did come out to a client, he told me that he was gay too so there was no problem.

I truly believe that the business case for diversity is a really good one and although we still need to make more progress, I believe that the UK is a very good place to be if you're gay.