Image caption Trafficking victims allege they were taken to the Kidney Centre for tests

"We will remove your kidney, and you will receive 300,000 rupees [£2,300]."

Sadi Ahmed was held hostage for three months by an organ trafficking gang.

In October last year, he was one of 24 people rescued by police in Rawalpindi, Pakistan.

They had been imprisoned in a building in an affluent suburb, awaiting the forced removal of their kidneys.

Three people are due in court later this month.

They deny all involvement in illegal transplants and trafficking.

Police say victims were lured to Rawalpindi in the hope of getting jobs.

They even were tricked into going to court, under the pretence of getting them documentation to work.

In fact, the gang was creating a paper trail to provide a cover story.

Their victims were held captive, for months in some cases.

Mr Ahmed told the BBC's File on 4 programme that he was taken to a commercial building, had his phone taken from him, and soon realised there was no job.

"There were 20 to 25 other persons sitting. I was told to shut up and be quiet and sit there.

"About 10 minutes later, the agent arrived and said get ready as I was going in for a test.

"I asked, 'What type of a test are you taking me for? What type of work are you offering?'."

The traffickers wanted to test his kidney, and told him he would be given the equivalent of £2,300 for the organ.

Mr Ahmed says he was "beaten up, not allowed to go out, we were padlocked in".

"We were threatened that the police would beat us up and we would be killed."

When police raided the building, Mr Ahmed was saved just in time.

He was due to have his kidney removed at a nearby hospital, called the Kidney Centre, a few hours later.

Image caption Sadi Ahmed says he and his family were left penniless after he was held hostage for three months by organ traffickers

Though happy to be free, while he was held captive his wife and four children struggled to survive without him, and built up debts.

He said: "I had my own property, it was taken away due to the debts. We are penniless now. We have lost our home."

Police officer Yasir Mehmood says the victims were "very weak and very sad" when he and his colleagues found them, locked behind a grille.

Dr Mirza Naqi Zafar, general secretary of the Pakistan Transplantation Society, says despite a ban on commercial transplants in 2010, there has been a resurgence in the illegal trade in recent years, with as many as 100 illegal transplants happening every month.

Transplant tourism

He says many of the operations are linked to transplant tourism, with wealthy foreign patients travelling to Pakistan for treatment.

This is driven by a global shortage in organs for transplantation, which allows traffickers to fill the gap between demand and supply.

Dr Naqi Zafar says black market prices for procedures are in the range of $50,000 to $60,000 (£48,000) per patient.

If donors are paid, they receive only a tiny proportion of that.

Image caption Pakistani police found 24 people imprisoned in this building awaiting the removal of their kidneys

Zafar Shahab, a man in his 50s, says he had a kidney removed without his consent at the hospital a year ago.

He was told he needed an operation for his own health problems, and "did not have the faintest idea" one of his kidneys would be removed.

When he returned home, his health deteriorated.

He says: "I am not working , I have no work. I cannot even lift 5kg (11lb), what work will I do?"

Dr Naqi Zafar is part of an informal intelligence gathering network of medics around the world, attempting to stem the illicit trade.

They received emails from the UK, Kuwait, Saudi Arabia, Australia and Canada, reporting that people of Pakistani origin had received transplants at the Kidney Centre and returned home with complications.

He says more needs to be done, calling for a proper system of investigation and intelligence, and that other countries could do more.

He says: "We need to talk to the countries where people are coming from, as to how they can help stop this organ trafficking".

The UK is signed up to the Council of Europe Convention Against Trafficking in Human Organs, and the Department of Health told us there were systems in place to prevent organ trafficking in the UK.

But NHS doctors have told us they had seen desperate patients go abroad to buy kidneys.

Data from NHS Blood and Transplant (NHSBT) shows that Pakistan is the number one destination of choice for patients seeking a kidney overseas.

Overall, about 400 people have received follow-up treatment in the UK after having had transplants overseas, since the year 2000.

Medics say cases are likely to be under-reported, and the figures do not include those patients who do not come back, or do not survive the operation.

Image caption Prof Vassilios Papalois has seen "catastrophes" following transplant tourism, and says we should be doing more "to make sure that people are not so desperate" they buy organs on the black market.

Vassilios Papalois is professor of transplantation surgery at Imperial College London.

He has seen several patients who have needed treatment after having transplants overseas.

One patient, a man in his 20s with a wife and young child, borrowed money on a credit card to pay for the operation in Pakistan.

The kidney failed and had to be removed, and he developed a serious infection.

He was given large quantities of the wrong antibiotics, leaving him deaf.

Prof Papalois says when the patient ran out of money, he was put on a plane and sent back to Heathrow.

He says the hospital had failed to treat the patient's sepsis, "simply because he didn't have enough money to pay them to do it".

Despite efforts to save him, the man died a few days later.

Doctors say cases such as this this are fuelled by the desperation of waiting for a transplant, and there can be particular problems for British Asian patients, who have a higher chance of developing kidney problems and face longer waits for transplants.

Image caption Adnan Aaftab has been on dialysis for five years and says a kidney transplant would transform his life, giving him "a future"

Patients from Black and minority ethnic groups make up a quarter of those on the transplant waiting list, and wait over six months longer for a transplant on average than white British patients.

NHS Blood and Transplant is trying to tackle the problem.

Last year saw the highest ever numbers of donations overall after death.

Four years ago, the Taking Organ Transplantation to 2020 strategy was launched.

It aims to bring the consent rate for organ donation after death to 80% by then.

It is currently at 62% overall, but for ethnic minorities the rate is half that.

However, Sally Johnson, NHSBT's director of organ donation and transplantation, says the number of people from minority communities receiving a transplant has increased.

In "2010-11 about 500 black and Asian people received transplants, last year that increased to 792, that's a big increase over the time period", she says.

File on 4: The Cost of a Kidney is on BBC Radio 4, 24 January at 20:00 GMT - catch up on BBC iPlayer Radio.

Have you got something you want investigating? We want to hear from you. Tweet us, or email fileon4@bbc.co.uk