The “special mini adventure” will form two four-minute episodes, titled ‘Space’ and ‘Time’, starring Smith alongside regular cast members Karen Gillan and Arthur Darvill.

Smith told TV & Satellite Week: “It’s great. The set-up is that the three of us are all in the TARDIS together, and the TARDIS materialises within itself and there are two Amy Ponds. It’s a real jaunt and a bit of fun.”

It was first reported last month that the special will feature two Amy Ponds. Asked to reveal more details, Smith commented: “I couldn’t possibly say, but she is quite pleased to meet herself, put it that way. Rory is bemused by the whole affair but, needless to say, two Amy Ponds in his life is no bad thing.”

He added: “Karen really enjoyed playing two versions of herself and it is a really funny performance from her. We are always laughing during filming anyway, but it was really nice to get to do a sitcom-like story in the TARDIS. Sometimes it is good to do something out of context and stand-alone and not have a full story narrative to follow.”

Red Nose Day 2011 takes place on Friday 18th March. The charity helps to transform the lives of poor, vulnerable and disadvantaged people across the UK and Africa.