A billboard advertising campaign was launched across central London this week by campaigners critical of the legalisation of gay marriage.

The billboard shows two men walking hand in hand while a child stands isolated.  The strapline says "But what about Sophie, Mr Cameron? ... Evidence shows children do best with married birth parents".

The campaign is being run by the Gay Marriage No Thanks group and is based on evidence from research that the best environment for raising children is within a family headed by their married biological parents.

The campaign group says there has been an "extraordinary lack of concern" shown for the welfare of children in the same-sex marriage debate in Parliament.  

"Children are at the heart of marriage yet the debate has been about only the interests of adults," said Alan Craig, spokesman for the Gay Marriage No Thanks group.

The billboard was driven around London to coincide with the Third Reading debate and vote on the Marriage (Same-sex Couples) Bill in the House of Lords.  

The Lords gave its approval to the Bill after a debate on Monday, meaning that gay marriage could be law by the end of the week.