Media playback is unsupported on your device Media caption Freedom to Marry founder, Nick Herbert: "There are many people who would like to enter into a marriage not just a civil partnership"

A campaign group backing same-sex marriage has been launched by senior Conservative MPs and peers.

In a Sunday Telegraph letter the group, which includes Education Secretary Michael Gove, said religious bodies should be able to conduct gay marriages but should not be compelled to do so.

Plans to legalise same-sex marriage have divided the Conservative party.

Ministers will respond to a consultation on how to introduce equal marriage later this week.

Prime Minister David Cameron, who supports the move, has promised a change in the law in England and Wales by the next election in May 2015.

Speaking in favour of same sex marriage, Mr Cameron has said he does not want gay couples to be excluded from a "great institution".

However, more than 100 of his MPs are thought to be against the idea.

Some have accused Mr Cameron of being out of touch with mainstream public opinion and fear the bill will alienate natural Conservative voters.

Labour leader Ed Miliband and deputy prime minister Nick Clegg also support gay marriage in church.

Civil partnerships are not marriages, which express a particular and universally understood commitment Sunday Telegraph letter

'Universally understood'

Mr Gove and London Mayor Boris Johnson are among 19 senior figures who have signed the letter calling for marriage to be open to all, regardless of sexuality.

The group, known as Freedom to Marry, was put together by former police minister Nick Herbert.

It includes Transport Secretary Patrick McLoughlin, who is Catholic, Foreign Office minister Alistair Burt, an evangelical Christian and leader of the Scottish Conservatives, Ruth Davidson.

The signatories argue that religious groups should be allowed to conduct same-sex marriages if they choose, but none should be compelled to do so.

In the letter to the Sunday Telegraph, they wrote: "Marriage should be open to all, regardless of sexuality.

"We recognise that civil partnerships were an important step forward in giving legal recognition to same sex couples.

Media playback is unsupported on your device Media caption David Cameron: "I don't want gay people to be excluded from a great institution"

"But civil partnerships are not marriages, which express a particular and universally understood commitment."

And, drawing on his personal experiences, Mr Herbert wrote in the Sunday Telegraph: "Civil partnerships for gay couples were a great step forward. Entering into one was the most important thing I have done in my life.

"I will forever be grateful for the courage of politicians - including many Conservatives - who, against much opposition, supported this change.

"But civil partnerships are not marriages. They convey almost the same legal rights, but they do not express the same universally understood commitment."

Speaking to Sky News, Mayor of London Boris Johnson called for speedy legislation: "The Labour party would vote for it, the Liberals would vote for it, huge numbers of Tories would vote for it - whack it through, let's knock this thing on the head.

"In a way I think it's a shame that the subject continues to be so kind of prevalent. Let's get it done and then let's talk about the real Conservative things we want to do."

Human rights challenge

Under government proposals, due to be developed and then published on Thursday by Equalities Minister Maria Miller, churches and other venues will be allowed to "opt in" to holding civil marriage ceremonies.

Ministers will offer a guarantee that no institution will be forced to marry gay people on their premises.

However, Conservative MPs and religious groups have questioned whether it would stand up to challenges under the Human Rights Act.

Prominent Tory MPs including Environment Secretary Owen Paterson, Defence Secretary Philip Hammond and backbencher Stewart Jackson have said they cannot support gay marriage, and a number of MPs have signed the "Coalition for Marriage" petition opposing it.

Conservative MP for Monmouth David Davies called plans to allow same sex marriages politically "barking mad," telling BBC Radio Wales the party could lose a "large number of very loyal activists" if it went ahead.

Media playback is unsupported on your device Media caption MP David Davies explains his concerns to Vaughan Roderick of BBC Radio Wales

The Church of England and Roman Catholics are also among those who have voiced opposition to same-sex marriage.

But some denominations and religious groups, such as Quakers, Unitarians and Liberal Judaism, support gay marriage and are expected to apply to be allowed to perform the ceremonies.

A vote on the issue is due in the spring and Conservative MPs will be free to vote according to their conscience.

Labour and the Lib Dems have not decided whether to grant their MPs a free vote, although the majority of MPs in the three parties are thought likely to back it.

Scotland already has plans for a bill to allow same-sex marriage with civil and religious ceremonies. The policy was announced by the Scottish Government in 2011, following a public consultation on the issue.

There are no plans for similar legislation in Northern Ireland.