Tamzin spent a month in the U.S. on a dolphin research programme, which included visits to sea-life centres and helping disabled children swim with the animals as part of their therapy.

'I really did get attached to the children,' she said. 'I was spending all day with them, and I was hanging out with their parents and brothers and sisters.

'Just the enjoyment that came from the children when they were swimming, regardless of their progress, made my heart flutter.

'I was spending time with people who were making a difference for the next generation. It made me, as an actress, feel quite small and want to do something a little bit more worthwhile.'

Miss Outhwaite, 31, was voted Britain's sexiest female soap star, and runner-up to Nigella Lawson for the title of sexiest woman on TV. But both she and the BBC insist the documentary was not just a vehicle for fans to see her in a series of fetching swimming costumes.

'This is all about research into completely wild dolphins,' she says of the Wild Dolphin Project which saw her and 11 others sailing 40 miles off the Bahamas in a spartan catamaran.

'The group sail out and spend days waiting for dolphins to show up. 'They swim with them to observe them. There is no touching allowed. The aim is to experience what they really do in their own environment, with as little influence from man as possible.

'We didn't wear scuba gear, just face masks, flippers and snorkels. Out there, learned to give the dolphins a lot of respect - not to go to them, but just to wait.

'They are as intrigued by you as you are by them. They come up and give you eye contact. And they play with you. They are like kids.

'They seem to be mimicking you a lot. You are in there on their terms, in their water, so when they have had enough, they leave. But they seemed to like being around humans, swimming in and out of us.

'I wasn't expecting the dolphins to be quite so tame. They were playful. I dived down and they mimicked me, and we were copying each other.

'It was so exhilarating and at the same time it provides you with this inner calm and peace because you're in their world and you can't hear anything but their clicking noises and squeaking.'