A survey of 450,000 UK adults has found that 1.5 per cent were willing to identify themselves as lesbian, gay or bisexual.
The Integrated Household Survey, carried out by the Office for National Statistics, is the second largest after the census.
This is the first time the survey has asked about sexual identity and the ONS stressed that the question was "experimental".
Nearly four per cent of those asked refused to answer, said they did not know or described themselves as "other".
Of the five per cent who did not say they were heterosexual, one per cent said they were gay or lesbian, 0.5 per cent said they were bisexual and 3.5 per cent refused to answer the question, described themselves as "other" or said they did not know.
Gay rights charity Stonewall and the government both use a figure of six per cent of the population being lesbian, gay or bisexual, which works out at 3.6 million people.
This figure comes from 2005 research by the Department for Trade and Industry.
Other studies on sexual orientation have found that the figure varies between six and ten per cent.
In this study, the ONS used the phrase 'sexual identity' rather than 'sexual orientation'. No responses were collected by proxy (allowing, for example, another member of a household to answer).
Stonewall welcomed today's figures but said they must be treated with "caution".
Ruth Hunt, deputy director of public affairs at Stonewall, said the charity was pleased the research had been carried out but said it was a "shame it took so long".
She told PinkNews.co.uk: "Six per cent is the Treasury actuary figure. Based on this, the figure is still about right.
"We have to view these results with caution. It's the first time people have been asked and we expect the figures to rise in a few years."
Ms Hunt added that such data should be collected as a matter of course, including in the census, and said Stonewall had urged GPs' surgeries to ask patients about their sexual orientation.
On the danger of the 1.5 per cent figure being used to argue against gay equality, she said: "We know other equality strands such as faith have this problem [of surveys not being representative]. Even the figures for faith do not reflect the lived experience of those on the ground."
The largest numbers of gay, lesbian and bisexual people were found in London, while the lowest numbers were in Northern Ireland.
Men were twice as likely as women to describe themselves as gay/lesbian.
The research also asked about religion, with 71 per cent of people describing themselves as Christian and 21 per cent saying they had no religion.
The research carried out between April 2009 and March 2010 and comprises the results of six ONS surveys.
Participants were telephoned or presented with cards asking which of the following options best described how they see themselves: heterosexual/straight, gay/lesbian, bisexual or other.
Ninety-five per cent said they were heterosexual, one per cent said they were gay or lesbian and 0.5 per cent said they were bisexual.
Just under three per cent stated "don’t know" or refused the question, one per cent did not provide a response and 0.5 per cent defined themselves as "other".