Health officials fear a huge drop in potential organ donors, bringing the threat of longer queues of those awaiting transplants.
A snapshot audit of hospital intensive care units between April 1 and June 30 last year suggested that 49% of relatives of patients who died and whose organs could be transplanted refused to give permission.
Previous studies in the 1980s and 90s indicated 30% of relatives would not allow organs to be used for transplants. The government agency UK Transplant is "very concerned" by early results from the most comprehensive survey of potential donors, involving 256 hospitals.
Campaigners for change - making people opt out of donation rather than opting-in through a register and organ cards - suggested the figures strengthened their position. The believe that proper debate about donation would enable people to make informed choices long before grieving relatives were confronted with hurried decisions.
Liberal Democrat MP Evan Harris, the party's former health spokesman, who revealed the UK Transplant figures, said: "We have had 20 years of government initiatives promoting opt-in, all worthy, none of which have resulted in a higher donation rate ... We cannot take the chance of waiting 10 years on a government promise of more of the same while rejecting presumed consent."
Michael Wilks, chairman of the British Medical Association's ethical committee, said: "If this refusal rate is consistent, you only have two solutions - you put the vast majority of people on the register or carry cards or you have presumed consent.
"Given the inertia people have around carrying cards, these findings, if maintained, make an absolutely unanswerable case for presumed consent." That could potentially double donations.
Reasons given for refusal by relatives in the survey included patients' wishes not to be a donor. More often, however, relatives were not sure what patients' views were, were themselves divided, or did not want their loved ones to undergo more surgery.
In Spain where each hospital has an official to chase up potential donors, the refusal rate is 24%, and the government plans to follow this example.
If Britain could attain lower European refusal rates, UK Transplant estimates 680 more kidney patients, 75 more heart patients and 320 more liver patients would benefit from transplants each year.
The Department for Health ruled out a move to presumed consent, saying there was little support from patients, doctors or researchers. Its human tissues bill, which threatens up to three years imprisonment for those who remove or retain organs without consent, offered a framework to renew confidence in donation. It plans to have 16 million registered donors by 2010, instead of the present 10.9 million.
The bill, whose second reading takes place in the Commons today, follows scandals such as Alder Hey and Bristol Royal Infirmary where children's organs were taken for research without parents' consent.
About 400 people on the waiting list die each year, and the gap between donations and those requiring replacement organs is growing as medical techniques improve, the population ages and organ-damaging diseases multiply.
More than 5,800 patients are registered for transplant, most awaiting kidneys, while there are only 2,800 transplant operations each year using organs from 2,675 donors - most are dead, although some kidney, liver and lung transplants involve living donors.
The survey results showed the refusal rate in Northern Ireland was 63%, yet in Scotland where 22% of the population were on the donor register, well above the 18% UK average, only 37% of relatives refused donation.
London and the north-east of England also recorded high refusal rates, 54% and 58%, while it was around 40% in Wales, south-west England and in much of the south-east.
