The twins are thought to be the only people with the condition

Catherine and Kirstie Fields, 11, from Llanelli, have Fields condition, making movement difficult and forcing them to spend much time in wheelchairs.

On Thursday they beat more than 50,000 entrants for a UK award run by That's Life magazine for the best smile.

Doctors remain puzzled by the condition which began when they were four.

They started to have problems walking and by the age of nine the girls needed frames to help them move.

They keep me and my husband going because they are smiling all the time - if you are feeling down, you just need to look at their faces

Lyn Fields

Now aged 11, they spend much of their time in wheelchairs. The condition also causes their hands to shake, making it difficult for them to write.

The girls' mother, Lyn, 37, said: "As they have been getting older, they have been deteriorating.

"They've had a lot of tests, where their blood tests have gone to Cardiff and Belgium, and their scans have gone to America. They have been tested for all kinds of conditions and they don't match anything.

"But it's not this affliction that makes my beautiful daughters unique. It's the way they carry on without a hint of self-pity.

"Catherine and Kirstie have grown to be bright and chatty girls. There's certainly nothing wrong with their minds or their sense of humour."

Lyn Fields with her twin daughters Kirstie (L) and Catherine

Mrs Fields, who nominated the girls for the award, said their smiles, cheerfulness and courage had helped to keep the family going while they sought a diagnosis.

She said: "They keep me and my husband going because they are smiling all the time. If you are feeling down, you just need to look at their faces."

Kirstie said: "I'm quite happy but mummy is normally sad. So I comfort her and she feels better then.

"When we are going to see a doctor, she cries and not us. We have to tell her not to cry."

Doctors cannot say how the girls' condition will develop in the future. But for the moment, all Catherine and Kirstie are interested in is deciding how to spend the £1,000 prize money their smiles have won them.

Mrs Fields said nobody could tell the family what the future will bring.

She said: "We can't think too much about it. We've just got to enjoy now, make the most of now and enjoy it. And that's what they're doing."