Over the past nine months, members of the organ donation task force have been considering whether Britain should adopt “presumed consent” - whereby all people are potential donors unless they actively opt out. Each of them, myself included, is committed to increasing the number of donors. All started with an open mind, with many - again I include myself - leaning towards the idea. But when we had reviewed everything we had heard, every single member opposed the introduction of presumed consent.

Why? In a word, evidence. This indicated that presumed consent would not increase the number of donors and might do the opposite. And increased numbers are the bottom line, not whether the public are “ready” for presumed consent - the bizarre suggestion of those Sunday clairvoyants who commented on the report before it appeared.

There was no single piece of evidence that tipped the balance. Rather, it was layer upon layer of challenge, concern and practicality that decided the case for us.

One issue ran through every discussion: trust. Trust that our organs will not be taken until we are really dead, or that treatment for the critically ill will not be pursued less vigorously if they are seen as potential donors. These fears are misplaced but they are primeval, akin to those we have about being buried alive. They were frequently expressed at public events. Trust that doctors act at all times in the patient's best interest is essential. Clinicians with the most day-to-day contact with bereaved families were particularly concerned about the erosion of trust presumed consent might bring.

Trust in doctors took a battering after the scandals at Alder Hey and Bristol Children's Hospital. The Human Tissue Act was introduced to put consent at the centre of medical practice. Presumed consent would go against its principles.

If we wanted to opt out would we trust the State to safeguard our personal data or to enact our wishes? From the comments at the public events, the answer is “no”. Lack of trust forced the repeal of presumed consent legislation in Brazil and badly dented confidence in the French system after a single highly publicised case of mistaken organ removal.

There are formidable practical problems. Being a donor is seen as the correct moral choice, so those who opt out might not want others to know. Such sensitive data requires greater security than the Organ Donors Register (ODR) can offer at present. This security is important given that a vocal minority think that those who opt out should get lower priority for transplants. If this view were widely expressed people might not let their true wishes be known.

Letting people know that they had the right to opt out would require an extensive and expensive advertising campaign that concentrated on a negative. Meanwhile, how does one ascertain the wishes of those who lack capacity, the homeless, visitors to Britain or non-British EU citizens who are entitled to NHS transplants?

While some are happy to donate solid organs such as kidneys, they may be iffy about hearts and downright squeamish about eyes. An “opt-out” scheme would have to record preferences: do you publish the whole list of some 70 transplantable tissues? People should have proper information but a list that feels like a butcher's order book would risk prompting many to say “no” to the whole lot.

Ethnic minorities are doubly disadvantaged by higher rates of diseases such as diabetes that may make transplants necessary and by having fewer matching donors. They account for 23 per cent of the donor waiting list, yet only 8 per cent of the population. The need to increase numbers within these communities is particularly pressing. All 17 faiths interviewed support organ donation yet their own members may be unsure of their faith's position. One study in Birmingham found that 60 per cent of Muslims believed organ donation went against their faith.

The families of donors said that it was important for them to take the decision, however hard, themselves. The families of recipients said that knowing that an organ had been freely given was a great comfort.

The public assume that presumed consent will increase donor numbers. But Spain had the system for a decade without increasing donation rates. What made a difference was when, in 1989, it began to develop a transplant co-ordination network.

According to the architect of the Spanish system, Rafael Matesanz, public confidence in the medical profession, understanding of the donation process and a professional approach to bereaved families are the key factors, not the law.

What will increase donation is removing uncertainty. In the UK only 10 per cent of families refuse permission if a relative's name is on the ODR but at least 39 per cent do if it is not. Merely by discussing organ donation in the media, more than a million names have been added to the register in the past year. There are surer ways to increased donation than presumed consent. Let's take them fast.

Vivienne Parry is a science writer and broadcaster and member of the organ donation task force

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