He said: "I just want to say I am absolutely determined that this Coalition government will follow in that tradition by legislating for gay marriage in this Parliament."
The commitment would appear to be at odds with suggestions in May that he would allow MPs a free vote on gay marriage - making its progress to the statute book by the next election in 2015 much more uncertain.
It will also frustrate senior Conservatives and many of the party's members who believe the policy is a Liberal Democrat-inspired distraction from more urgent tasks, such as addressing unemployment and securing the economic recovery.