The author of new research into organ trafficking has called for a concerted international effort to confront the problem.

Dr Ana Manzano, of the School of Sociology and Social Policy at the University of Leeds, says a combination of factors means nobody knows definitively how many organs are being traded across the world.

She said: "Unless these issues are addressed and countries work together to take firm action against the traffickers, more people who have their organs trafficked will die.

"Even in the UK, although the World Health Organization has identified us as a buyer country, we don't know the full extent of it."

The research, published in the journal Transplantation, describes five main reasons why the true extent of organ trafficking is difficult to pinpoint.

These factors are:

A reluctance by those who give away their organs to talk, because of fear of prosecution.

No agreement between countries about what penalties should be in place for those who buy organs, and little consistency in enforcing laws.

The high status of surgeons. Some surgeons perform illegal transplants knowing that they will only be caught if they are reported to regulatory bodies by colleagues.

The nature of organ trafficking offences means they can span several countries, making tracing organs difficult.

Insurers play a part in the proliferation of organ trafficking by paying for follow-up treatment to transplant patients.

Dr Manzano added: "Together, these factors have helped create the practice of organ laundering -- where the illegal purchase of organs takes on the veneer of a legal transaction.

"Countries should follow the example of places like Spain where reporting the recipient of an organ purchased abroad is compulsory if follow-up care is requested."

Although there is no internationally agreed definition, 'organ trafficking' is broadly defined as situations in which people are tricked into giving up organs, may sell them for financial gain but are not paid for as agreed.

Dr Manzano added: "If countries do nothing about this problem, the consequences for both donors and recipients can be terrible, as they may have to deal with dreadful health outcomes."