Gordon Brown has thrown his weight behind a move to allow hospitals to take organs from dead patients without explicit consent.

Writing in The Sunday Telegraph, the Prime Minister says that such a facility would save thousands of lives and that he hopes such a system can start this year.

The proposals would mean consent for organ donation after death would be automatically presumed, unless individuals had opted out of the national register or family members objected.

But patients' groups said that they were "totally opposed" to Mr Brown's plan, saying that it would take away patients' rights over their own bodies.

There are more than 8,000 patients waiting for an organ donation and more than 1,000 a year die without receiving the organ that could save their lives.

The Government will launch an overhaul of the system next week, which will put pressure on doctors and nurses to identify more "potential organ donors" from dying patients. Hospitals will be rated for the number of deceased patients they "convert" into donors and doctors will be expected to identify potential donors earlier and alert donor co-ordinators as patients approach death.

But Mr Brown, who carries a donor card, has made it clear he backs an even more radical revamp of the system, which would lead to donation by "presumed consent". The approach is modelled on that of Spain, which has the highest proportion of organ donors in the world.

"A system of this kind seems to have the potential to close the aching gap between the potential benefits of transplant surgery in the UK and the limits imposed by our current system of consent," Mr Brown writes.

He voted against such a system in 2004 - but sources close to the Prime Minister said last night that the measure proposed then was a much harder version of his latest plan, without families having the final say.

Patients' groups said that they were appalled by Mr Brown's intervention. "They call it presumed consent, but it is no consent at all," said Joyce Robin, from the watchdog Patient Concern. "They are relying on inertia and ignorance to get the results that they want." She said that the

Government had made little effort to get people to register to give up organs after death. "Where is the big media campaign, where are the leaflets? Why, when I go to see my GP, doesn't he ask me about organ donation? These are the things they should be doing - not taking away our right to decide what happens to our bodies."

Katherine Murphy, of the Patients Association charity, agreed. "We don't think a private decision, which is a matter of individual conscience, should be taken by the state. If people want to give the gift of life, that is their right, but it must be something that is a voluntary matter. "

While polls show 90 per cent of Britons are in favour of organ donation, 40 per cent of relatives refuse consent for the organs of their relatives to be donated, a figure which rises to 75 per cent among black and ethnic minorities. To solve this, the organ taskforce plans measures to boost donation, including putting pressure on doctors to identify patients as potential donors before they have died.

The taskforce report - to be released on Tuesday - calls for a senior doctor to be appointed in every hospital as a "champion" of donation, along with a lay person to spread the message about the importance of donation locally.

The force, which is to publish a report on "presumed consent" this summer, hopes its 14 recommendations will lead to 50 per cent more donations in five years.

It admits to a possible "conflict of interest" between medical staff, trying to save lives and those keen to ensure every possible organ is harvested. Dr Kevin Gunning, an intensive care consultant at Addenbrooke's Hospital, Cambridge, and a member of the UK Transplant's advisory group, said the measures could put doctors and relatives under pressure. "If, as a doctor you have turned your thoughts to your patient being a donor when they are still living, that is a real conflict."

Dr Bruce Taylor, of the Intensive Care Society warned that early indicators of death were not reliable. "The only way to be sure is to do all the tests which show brain stem death; anything in advance of that is only a prediction."

But Chris Rudge, of UK Transplant, the authority in charge of organ donation and transplant, insisted patients would not be considered as donors at any point where survival was possible.