RECs review research proposals and give an opinion about whether the research is ethical. They also look at issues such as the participant involvement in the research. The committees are entirely independent of research sponsors (the organisations responsible for the management and conduct of the research), funders and the researchers themselves.
The majority of research will require a positive REC opinion. However, some research carried out in the context of the NHS is exempt from the requirement for a NHS research ethics committee opinion. See http://www.hra-decisiontools.org.uk/ethics/ to determine whether you need to apply for a NHS REC opinion. Different types of NHS RECs review different types of research proposals, and there are full details on the Health Research Authority (HRA) website (given below).
Your research may require a different type of ethical review. These are provided by the Gene Therapy Advisory Committee, the Social Care Research Ethics Committee, the Ministry of Defence Research Ethics Committee and the Higher Education Institution (HEI) Research Ethics Committees. Your study sponsor will be able to advise which the appropriate committee is.
For detailed information for researchers about REC approval and the types of NHS RECs, please see the HRA website, here: https://www.hra.nhs.uk/approvals-amendments/what-approvals-do-i-need/research-ethics-committee-review/.