Procedures to create GM babies with three or four parents will be allowed to take place in the UK, the fertility watchdog has said.
Parliament backed the controversial move last year and the Human Fertilisation and Embryology Authority (HFEA) today gave its formal approval.
The Christian Institute warned that problems with the procedures may only arise decades from now, as the techniques require editing the human germ-line.
Last month, a scientific panel told the HFEA that it believed the procedures should go ahead – despite acknowledging safety concerns.
It was not possible, the panel said, to rule out the risk of faulty mitochondria increasing as the children grew up.
The panel recommended that due to the experimental nature of the technique, patients should be made aware “that there can be no guarantee of safety and efficacy”.
Humphrey Dobson, Deputy Director of The Christian Institute, said: “There are very real safety fears because the genetic changes will be passed down the generations. No one knows if this is safe.
“These plans do not cure anyone alive today suffering from mitochondrial disease. Instead they seek to create new ‘disease-free’ children.
“We should be putting resources into adult stem cell treatment which offers hope for treating mitochondrial disease.”
Dr David King, from the Human Genetics Alert group, said the decision opened the door to designer babies.
Sally Cheshire, who chairs the HFEA, called it a “historic” decision but claimed it was giving it a “cautious” rather than a “gung-ho” go ahead.
Parliament’s decision in February 2015 to back the proposals was taken after a short debate on the issue.
MPs from across the political parties criticised the time allowed, with former DUP MP Dr William McCrea saying there was “general dismay” that only 90 minutes was allocated for a “decision of such magnitude”.