A shortage of organs contributed to the deaths of hundreds of patients on the organ donation waiting list last year, figures show.
Three families a week were saying no to organ donation because they were unsure of their relative's wishes, NHS Blood and Transplant (NHSBT) said.
It said that when families have to decide on their relative's behalf, some think it is safer to say no.
The reluctance to talk about the issue is contributing to a deadly shortage of organs, with 457 people dying whilst on the waiting list last year, NHSBT said.
Organ Donation Week begins today, and the organisation is encouraging families to discuss the issue.
Currently in England there is an opt-in approach to organ donating, meaning a person has to register their consent to donate their organs in the event of their death. A family member can make the decision if the person is not registered.
Wales introduced a soft opt-out system in 2015 which means everyone who doesn't actively to opt-out is seen as giving 'deemed consent' for their organs to be donated upon their death. They can also choose a representative to make the decision.
Four-year-old Aoife O'Sullivan died in March 2016 while waiting for a heart transplant, and her parents have urged people to start discussing the topic.
The youngster was in need of a new heart after suffering heart failure from restrictive cardiomyopathy - a condition which made her heart muscle rigid.
After she died, her parents - Michelle O'Sullivan and Neil Forsyth - chose to donate her kidneys.
Anthony Clarkson, assistant director of organ donation for NHSBT, said: "It's a tragedy, hundreds of people are dying unnecessarily every year waiting for transplants.
"We know that if everyone who supported donation talked about it and agreed to donate, most of those lives would be saved.
"This Organ Donation Week tell your family you want to save lives. A few words now can make an extraordinary difference. It will also make things much easier for your family to make the right decision.
How do you become an organ donor?
People can choose to donate organs in the event of their death or - along with blood - people can donate their kidneys, liver, and tissue while they are alive.
Living donation requires major surgery and potential donors are carefully assessed to determine their suitability.
- Kidneys - Around a third of all kidney transplants in the UK are donated by a living person as a healthy person can lead a normal life with one
- Liver - Part of a liver from a living person can be donated because the liver can regenerate itself, although this is less common than living kidney donation
- Tissue - Those undergoing hip operations can donate part of their thigh bone, while amniotic membrane (part of the placenta) can be donated after caesarean section to be used in eye operations
Donating organs and tissue after your death can potentially help someone live or improve their health and quality of life.
You can choose to donate the following:
- Small bowel
The NHS "strongly suggests" people tell their family and friends whether or not they want to be an organ donor in the event of their death.
Visit the NHS Organ Donation website for more information.