The trust admitted gross failures in care and has also agreed to pay an undisclosed settlement to the couple.

Shattered

The couple, who have not been named, discovered the mix-up when they attended the clinic for the implantation of their last remaining viable embryo in December 2007.

It later emerged that the patient who had wrongly been given their embryo had been given the morning after pill when the mistake was spotted almost immediately. This resulted in the procedure being terminated.

"In less than 10 seconds our wonderful world was shattered when the senior embryologist stood in front of us and said, 'I'm very sorry to tell you, but there's been an accident in the lab. Your embryo has been destroyed'," the woman told the Mail on Sunday.

She added: "We were both rooted to our seats. We were stunned and trembling. We held each other tightly, and sobbed and sobbed."

The couple's solicitor, Guy Forster, said the couple were "absolutely distraught" by what happened.

"Even some time later, they still get very tearful when talking about what they went through, and I don't think that will leave them," he said.

The woman, a 38-year-old hospital worker, said the couple rejected an offer for a free round of IVF treatment as they felt they could no longer trust the hospital.

She added that the incident put a great strain on her relationship with her husband.

The couple began fertility treatment in 2000.

Following the third cycle of treatment, the woman became pregnant and in April 2003 gave birth to a son.

The remaining embryos were frozen and, in line with the clinic's policy, were kept for five years.

Wrong shelf

In November 2007 the clinic contacted the couple with the news that just one embryo had survived and was in good condition.

The couple decided to take this last chance to add to their family.

On 5 December 2007, they attended the clinic for the embryo to be transplanted, unaware that in the laboratory a trainee embryologist had mixed up their embryo after taking it from the wrong shelf of the incubator.

The trainee embryologist failed to carry out "fail-safe" witnessing procedures to ensure the embryo being taken from the incubator and implanted belonged to the correct patient.

The mistake was only discovered when another colleague later found that the correct embryo, that belonging to the couple, was missing from the incubator.

The Human Fertilisation and Embryology Authority (HFEA) was informed of the incident.

An independent inquiry into failings at a Leeds IVF unit in 2002 led to a series of recommendations being made to the HFEA and all fertility clinics to safeguard against this type of incident in the future.

Dr Sammy Lee, a fertility expert from University College London, said there were "good procedures" in place to prevent this kind of "disaster", but the potential for human error would always exist.

He said in good practice, not more than six couples' embryos should be stored in one incubator - and usually in six separate compartments. But he conceded that a really busy clinic may store more than one couple's dish in the same compartment.

'Safeguards'

Separate incubators for each embryos would help minimise the risk, he said, and some clinics were also introducing "IVF witness" - or electronic tagging.

"But you still have to remember someone has to put the chip on the dish, so what if someone puts the chip on the wrong dish?" he added.

The HFEA's effectiveness to act as a watchdog has recently been questioned by the head of that inquiry, Professor Brian Toft, following recent reports of IVF mistakes in London in February 2009.

Ian Lane, medical director of Cardiff and Vale NHS Trust, said: "We apologise unreservedly for this mistake.

"This was a rare but extremely upsetting incident for everyone involved and we take full responsibility for the distress caused to both couples and their families."

Mr Lane said that immediately after the event, the trust carried out an internal investigation to find out exactly how it happened, and how it could be prevented from happening again. The trust also notified the HFEA, who carried out their own external investigation.

He added that as a result of both investigations, the trust had made a number of improvements to its systems and checks, in line with the recommendations made in the reports.

He said: "We have strengthened our protocols and reduced our workload to relieve pressure on staffing levels. Working conditions at the unit have since improved and we are continuously monitoring the safeguards and procedures we have put in place."