Conjoined twin girls have been successfully separated the day after their birth during an emergency operation in London, their mother revealed on Thursday.

The chances of Rosie and Ruby Formosa surviving a separation were deemed low by doctors, but now the pair are "bubbly babies" who are expected to lead happy and normal lives, mother Angela said.

The identical twins, now 12 weeks old, underwent the life-saving operation at London's Great Ormond Street Hospital on July 27, the day after they were born. They were joined at the abdomen and shared an intestine.

Formosa, 32, from Bexleyheath in Kent, described her shock at finding out that the twins were joined.

Her first pregnancy, with daughter Lily, now five, was "textbook", she said -- but an early scan during her pregnancy with the twins showed that they appeared very close together.

She went for another scan and, between 16 and 20 weeks, discovered that the girls were joined. "I didn't know what to think, I was shocked and I felt sad," she said.

Doctors could not tell where the pair were connected, she said, adding: "We didn't know what to expect until they were born."

At 34 weeks the babies were delivered by Caesearian section at University College Hospital in London. Doctors assessed the twins and within hours they had been taken to Great Ormond Street, the renowned children's hospital which is one of the most experienced in the world in the treatment of conjoined twins.

Paediatric surgeon Agostino Pierro, leading the team of doctors operating on the girls, said they had been rushed into the emergency surgery because of an intestinal blockage.

"We are delighted with the outcome of the operation," he said. "The babies will need further treatment in the future, but we expect that they will both be able to lead happy and normal lives."

Formosa and husband Daniel were "happy and relieved" to have their daughters home. "They are normal bubbly babies who are starting to smile and cry when they want something," she said.

The couple described their gratitude towards hospital staff. "When I was pregnant they were saying the survival chances were quite low," Formosa said. "For them to have been operated on and doing so well -- it is amazing."