Most people would accept a donor organ if they needed one but only 27% are signed up to donate after death, research has shown.
A poll of 1,500 people not on the organ donor register found 45% had good intentions to join but had not yet got round to it.
Almost one in five (19%) said they needed to involve their family in the decision on whether to become an organ donor.
Separate research among almost 2,000 people suggested a gap between the number who think they have signed up to be a donor (35%) and the number actually on the register (27%).
A total of 16.5 million people are currently signed up, but ministers and health experts say many more are needed. More than 10,000 people in the UK currently need a transplant.
However, more than 1,000 people – three a day on average – die every year before they get one.
Lynda Hamlyn, Chief Executive of NHS Blood and Transplant, which is launching a new campaign on the issue, said: "Many people have told us that they just need a nudge to motivate them to join the organ donor 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."