According to a number of international law and human rights declarations, human germline modification such as the 3-person IVF techniques are considered unethical human experimentation and an abuse of human rights:
 

• According to the 2001 European Union Directive on clinical trials, "No gene therapy trials may be carried out which result in modifications to the subject's germ line genetic identity." For this reason, it may actually be illegal for the United Kingdom to conduct clinical trials of 3-person IVF, which could be why they have proposed to move it straight into fertility clinics instead.
   
• The United Nations Education, Scientific and Cultural Organization’s (UNESCO) Universal Declaration on the Human Genome and Human Rights indicates in Article 24 that “germ-line interventions” could be “contrary to human dignity.”

• The Charter of Fundamental Rights of the European Union in Article 3 prohibits "eugenic practices, in particular those aiming at the selection of persons."

• The Council of Europe’s Convention on Human Rights and Biomedicine indicates in Article 13 that “an intervention seeking to modify the human genome may only be undertaken for preventive, diagnostic or therapeutic purposes and only if its aim is not to introduce any modification in the genome of any descendants.”

• The International Covenant on Civil and Political Rights indicates in Article 7 that “No one shall be subjected without his free consent to medical or scientific experimentation.”
   

Additionally, more than 40 countries around the world have laws on their books prohibiting the inheritable genetic modification of humans. No country until the UK had laws to enable it. See here for the full list.

Legalization of one form of human germline modification could have a lasting impact on policy around the world. See "Potential impact of human mitochondrial replacement on global policy regarding germline gene modification" in Reproductive BioMedicine Online for more details.