In England, Scotland and Northern Ireland, your organs and tissue will only be used if you have given consent. Wales changed to "opt out" in 2015 and Scotland is considering it. England and NI rely on public awareness campaigns.
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In the first six months after the law change, there was a 38% reduction in the number of people in Wales waiting for a transplant. Just 36% of people in the UK are now on the NHS Organ Donor Register, compared with 30% five years ago. According to a recent BBC article http://www.bbc.co.uk/news/health-40564329 457 people died last year while on the waiting list, as they did not get a donor organ in time. 875 people became too ill to undergo transplant surgery.