When you sign an Organ Donor Card, You are making a legal contract with the relevant organisation.
Legally no one can override your wishes without it going to court.
However, It is policy with the Organ Donor Register (and others like Lifeline Wales etc) to ask permission from the family and if they don't agree then they won't pursue it.
It's a big problem really.........
When I die, if part of me can be used to help someone else live or have a better quality of life then I would like to think that will happen (after all, I won't have any use for them).
The problem comes when someone is grieving they may not be able to bear to make that decision.
Once what has happened has sunk in and they've had time to consider the situation, it's probably too late.
There were recently plans to make people opt out from being a donor rather than opt in but I think this has been since shelved.
The important thing at the end of the day is to let your family know how you feel and why, hopefully this will help them to make the right decision when the time comes.
Hope that helps
Danny