A petition launched in defence of traditional marriage has passed the half million mark.

More than 500,000 people have signed the petition from the Coalition for Marriage (C4M) calling upon the Government to retain the current definition of marriage between one man and one woman.

The petition has now been signed by more people than voted in the local elections in Birmingham or Manchester.

It was launched in opposition to the Government's plan to widen the definition of marriage to include same-sex couples.

A consultation into gay marriage was launched in March but the Government stated at the time that the purpose was to discern how to implement the change, not whether it should go ahead.

Colin Hart, Campaign Director of C4M, said the Government’s plans were "radical and profoundly undemocratic".

“Changing marriage is not like raising or lowering the rate of VAT. It would have profound implications at all levels of society, and for all age groups," he said.

“Marriage has been the bedrock of society for a thousand years. The Government should pause for thought before they unravel an institution that has served Britain so well.

“In today’s turbulent, fast-moving world, the continuity of marriage as a union between one man and one woman is something we should be celebrating, not changing.”

“This campaign proves that the British people reject the Government’s plans to change the definition of marriage. With half a million signatures, they must listen to their voters.”

The campaign group’s new milestone was reached as Chancellor George Osborne promised to "focus on the things that really matter".

Mr Hart welcomed the comments by the Chancellor and described them as a “watershed moment for the campaign to save marriage”.

Polls by ComRes have found a widespread lack of support for the Government's plans.

One survey conducted in the run-up to last week's local elections found that three-quarters of David Cameron’s constituents who voted for him at the general election oppose his plans to redefine marriage.

Sixty-five per cent of people in his Witney constituency agreed that marriage should continue to be defined as a lifelong exclusive commitment between a man and a woman.

Seventy-two per cent agreed that same-sex relationships should be legally recognised through civil partnerships rather than through redefining marriage.

More than half (56%) said that Cameron’s decision to make legalising same-sex marriage a priority left him “out of touch with ordinary voters”.

Andrew Hawkins, the Chief Executive of ComRes, said: “Perhaps the most disturbing finding for Conservative strategists, however, is that SSM plays particularly badly among large numbers of its disaffected 2010 voters.

“People who voted Conservative in 2010 but do not intend to now are ‘less likely’ to vote for that Party than ‘more likely’ on the basis of this policy by a ratio of just under three to one.

“In other words, while this policy encourages some 12per cent of former Conservative voters to be more likely to return to the fold, fully 32 per cent say it makes them less likely to do so.”

Mr Hart said the Government seemed to be accepting that they were wrong on gay marriage.

"The Government has instigated a consultation on their proposals, saying that the consultation was about how not if. Until now they have refused to listen to those who opposed redefining marriage. I hope that this flawed decision will now be reversed.”