Thanking the ministers and civil servants who helped to pass the legislation into law despite objections from the Tory Right, he said: It's been a real pleasure to work with you and to deliver this landmark social change for our country, which to me still comes back to the simple word of commitment."

He said that he wanted to "export" same sex marriage around the world so other countries could follow suit.

He said: "I've told the Bill team I'm now going to reassign them because, of course, all over the world people would have been watching this piece of legislation and we've set something, I think, of an example of how to pass good legislation in good time.

"Many other countries are going to want to copy this. And, as you know, I talk about the global race, about how we've got to export more and sell more so I'm going to export the bill team. I think they can be part of this global race and take it around the world."

In his speech he told guests four times how proud he was to have been Prime Minister when same sex marriage was legalised in Britain, describing the process of legalising gay marriage as "a long, tortuous parliamentary process".

He said: "I'm personally proud of this. I think I'm probably the only Conservative Prime Minister who's taken this step, but I'm very proud to have taken it. I think it's a really good step, and thank you for helping me to stick with the plan and get it done so quickly."

Mr Cameron said the move was as important to homosexual people and their parents.

He said: "A mum came up to me the other day in the street in my constituency and said, 'Why I'm so pleased about this is that I've got a straight son and a gay daughter, and I now know I'm going to be able to go to both of their weddings, and that makes me really happy'."

During the event Mr Cameron, together with his team of ministers behind the legislation, signed a copy of the Marriage (Same Sex Couples) Act 2013.

Among the guests were television presenter Clare Balding, and her partner Alice Arnold, a former BBC Radio Four newsreader. Other MPs and ministers who attended included Crispin Blunt and Alan Duncan.

Miss Arnold and Miss Balding are expecting to get married when the law finally allows. Afterwards, Miss Balding told The Daily Telegraph: "It was great - a very good speech by the Prime Minister. He said that he had delivered on a promise and he was very proud to have done so."

Asked when she would "tie the knot", Miss Arnold told The Daily Telegraph: "I expect so - not soon but we are not able to yet. You can rest assured that whenever we will keep it will be very quiet."