The shift to an opt-out organ donation system in Scotland could lead to fewer people being registered, not more, a Christian campaign group has warned.
CARE for Scotland said MSPs were taking a "real risk" by changing from the current opt-in donor register.
Under the new system, consent for organ donation will be automatically presumed by the state, meaning that individuals must opt out in order to remove themselves from the list.
MSPs voted by 116 to three in favour of adopting an opt-out system.
While they are hoping that the change will see an increase in organ donations and reduce waiting times, CARE for Scotland has warned that the new system could cause potential donors to proactively remove themselves from the register if they take offence at the idea of the state presuming consent.
It added that an opt-out "fundamentally alters the nature of organ donation" so that it is no longer a "genuine gift" on the part of donors.
CARE for Scotland national director Dr Stuart Weir said there was no evidence that moving to an opt-out system would necessarily increase the number of organ donations.
Wales introduced an opt-out law in December 2015 but a study two years later in December 2017 found that the new system had not led to an increase in donors.
Dr Weir said the Scottish Government needed to ensure that people were properly informed about what the change means.
He also called for more nurses specially trained in the field of organ donation to help patients understand their options.
"MSPs are taking a real risk here because we are talking about time and money being spent on radical shift in the way we do organ donation and there's no guarantee the change will work," he said.
"It is also highly possible potential donors, who are opposed to the state presuming their consent, will be put off by this change and will then opt-out in protest which would be a real shame and would not happen under the opt-in system, where organ donation remains a genuine, voluntary gift.
"Scotland already leads the way in terms of the number of people on the organ donation register and we are concerned this change of system could undermine this."