Laws governing organ donation and tissue retention are to be overhauled, possibly allowing more transplants.
The transplantation changes could help reduce organ shortages
Under the Human Tissue Act, people will have a legal right for their wishes to be followed, meaning doctors could over-rule relatives' objections.
People will also face up to three years in jail if they remove and store human tissue without consent.
The changes apply to England, Wales and Northern Ireland and were prompted by a variety of scandals.
Human Tissue Act
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Licenses needed for places carrying out post mortems and anatomy schools
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Chris Rudge, managing director of UK Transplant, said the wishes of approximately one in 10 possible donors were overturned by their families under the current system.
He said that, from Friday, the legal onus would be placed on the wishes of the deceased person - at the moment neither the relatives or deceased person has rights enshrined in law.
This meant that in practice if relatives objected to organ donation doctors had to respect their wishes.
Under the new law, doctors will now be able to insist on the organs being used where consent by the person had been given even in the face of objections from family members.
However, many believe if relatives object strongly doctors will not exercise this right.
Mr Rudge said: "Many more people could receive a life-saving transplant with the donors' wishes being given priority," he said.
The act will also mean donation from strangers, including "altruistic" donation, will now be allowed.
A new regulatory authority called the Human Tissue Authority (HTA) will oversee the act, and will issue licences to places carrying out post-mortems, anatomy schools, establishments storing tissue for research, and sites that display human tissue, such as museums.
The HTA said it expected to be issuing licences to about 500 establishments.
Under the legislation, consent must now be given for body parts, organs or tissue to be removed, stored or used for purposes including research and post-mortem.
Failure to do this could result in a three-year prison sentence and/or a fine.
Adrian McNeil, chief executive of the HTA, said: "Consent is the underlying principle on which this act is based.
"This is an important milestone for the HTA, and for patients, the families and professionals."
Health Minister Rosie Winterton said: "The HTA has played a key role in bringing activities involving the use of human organs and tissues into a regulatory framework that builds on best practice, in many cases for the first time."
The scandal at Alder Hey hospital prompted the changes
At Alder Hey hospital it was discovered that organs were removed from thousands of dead children's bodies without parental consent over a period of seven years, but it was often decades before parents realised what had happened.
David and Hazel Thewlis, who were affected by events at Alder Hey Hospital, said: "The Human Tissue Act has provided a new foundation for the people of this country.
"Let us all remember that consent was the cornerstone of the act, and this must colour all of the work of the Human Tissue Authority."
But some have criticised the act, and have said it will place an extra administrative burden on researchers and could possibly hinder the amount of tissue available for research.
Peter Furness, vice-president of the Royal College of Pathologists, said that there were good points and bad points regarding the act.
One problematic aspect was that the licensing was expensive, time consuming, complicated and over bureaucratic.