The government on Friday hoisted the rainbow flag symbolising gay pride over one of its ministries for the first time.
Deputy Prime Minister Nick Clegg requested the flag be flown on Whitehall, the central London street that houses several ministries, ahead of the World Pride parade celebrating gay rights in the capital on Saturday.
"There has to be a first time for everything," said Clegg, leader of the Liberal Democrat party.
"Flying this iconic flag in the heart of Whitehall is a small but important emblem that the government and this country are behind equal rights," added Clegg, whose centrist party is junior partner in a coalition led by the centre-right Conservatives.
The striped rainbow flag was raised over the Cabinet Office, the ministry responsible for coordinating central government.
Ministers have pledged to legalise gay marriage in England and Wales by 2015 despite opposition from some Conservatives and religious figures.
Clegg has gone further by saying religious venues such as churches should be able to host gay weddings.
Homosexual couples in Britain have been able to obtain civil partnerships, giving them similar legal rights to heterosexual married couples, since 2004.
But gay couples cannot hold religious wedding ceremonies and campaigners say that having civil partnerships rather than gay marriages perpetuates the idea that same-sex unions are less valid than heterosexual ones.
The Church of England and Roman Catholic Church have staunchly opposed the changes, fearing they could eventually be forced to preside over gay weddings.
The government has pledged to introduce safeguards to prevent religious groups from having to act against their conscience.