A total of 45% have the best intentions to sign up but have not got around to it.
Research also shows a disparity between those who think they have already signed up to be an organ donor (35%) and those who have (27%).
A multi-media NHS campaign launched this week asks if we would accept an organ, shouldn't we be prepared to give one?
More than 10,000 people in the UK currently need a transplant but an average of three a day die every year before they receive one.
Lynda Hamlyn, chief executive of NHS Blood and Transplant, said: ‘Many people have told us that they just need a nudge to motivate them to join the register and commit to helping someone live after their death.
‘By focusing on the gap between the number of us who say we would accept an organ if we needed one and the number who have actually joined the register, our campaign highlights the urgent need for those who believe in organ donation to act now.'
Register at organdonation.nhs.uk