Organ donation in the UK currently relies on the principle of informed consent.

Informed consent is an 'opt-in' model - people have to sign up to donate organs. In this model, donors must also be made aware of what they are signing up for and have the option of withdrawing their consent at any point.

They must also not face any inducements to donate organs - it is seen as an entirely voluntary, altruistic gesture that benefits others.

Only people aged 16 and over can consent to organ donation. For children, parents or legal guardians make decisions.

Some difficulty surrounds the ability of people with forms of mental impairment to make informed medical decisions. Generally, family members or legally specified individuals decide based on the presumed wishes of the donor.

Informed consent contrasts with presumed consent (the assumption that an individual is happy to donate organs after death unless he or she has explicitly said that they do not wish to).

Presumed consent is controversial because medicine is founded on the notion of patients having the right to choose what medical procedures are carried out on them. Some argue that this right should extend to disposal of the body or parts of it after death.

Others argue that medicine has a moral responsibility to patients in need of organs, and that the rights accorded a living person do not apply to a dead body.

In practice, presumed consent can be either 'hard' or 'soft': in a soft approach, relatives are approached and have the opportunity to request that organs are not used even if the deceased has not specifically opted out. This is the position recommended by the British Medical Association.

Spain is often held up as a country where opt-out systems have led to an increase in transplantation - it carries out around 34 transplants per million of population while the UK manages just 13 per million. A study carried out in 2006 suggested opt-out systems typically increase the numbers of transplants by around 25-30 per cent.

To support its system, Spain has established a sophisticated transplant network, which includes specially trained staff who liaise with families after a death. In particular, 'organ procurement officers' have the sensitive task of dealing with grieving relatives, to persuade them not to block the harvesting of organs. An insight into this key role can be seen in Pedro Almodovar's film 'All About My Mother' - the main character, Manuela, is a procurement officer.