The world's oldest living conjoined twins have defied doctors' predictions and are set to celebrate their 50th birthday this Sunday

George and Lori Schappell, who are joined at the head, are marking the landmark birthday with a trip to London.

Remarkably, the Siamese twins are able to live very different and separate lives, with Lori having had relationships and George � who was originally named Dori � deciding to live life as a man.

Lori is also a champion ten-pin bowler and George performs as a country and western singer.

"When we were born, the doctors didn't think we'd make 30, but we proved them wrong," the Daily Mail quoted Lori as saying.

"We have learned so much in the last 50 years and will continue living life to the full," she added.

While Lori, who is 5ft 1in, was born able-bodied, 4ft 4in George suffers from spina bifida, which has caused severe mobility problems.

The twins, from Pennsylvania in America, were born sharing 30 per cent of their frontal lobe brain tissue and critical blood vessels, meaning they cannot be separated.