A historic vote to legalise gay marriage will be held in the new year after David Cameron decided to make the reform a top priority.

The Prime Minister is to take on his right-wing Tory critics over the issue and – with the backing of his Liberal Democrat deputy Nick Clegg – will introduce the contentious legislation within months.

The Independent disclosed last month that Mr Cameron had told his ministers to prioritise the move, regarding it as a "straightforward matter of equality". However, the decision was not foreshadowed in this year's Queen's Speech and the Government is also yet to issue its response to a consultation on how the reform could be implemented.

But ministers will now find time in the Commons timetable in the new year to hold a free vote on the move. It is certain to be passed as it is supported by the vast majority of Labour and Liberal Democrat MPs – even though up to 100 Tory backbenchers could oppose the reform after being inundated with protests from party members – paving the way for the legislation to reach the statute book by the end of 2013.

Mr Cameron hopes to prevent the opposition campaign gathering pace. More than 600,000 people have signed a petition which insists marriage can only take place between a man and a woman.