The Channel Tunnel Safety Authority is an international regulatory body responsible for safety in the Channel Tunnel.
The CTSA was established by the Treaty of Canterbury. It advises the Intergovernmental Commission on safety matters, and ensures that safety rules in the Channel Tunnel are in line with prevailing safety laws.
The CTSA has five members from France, and five from the UK; leadership alternates annually.
After safety rule changes which would permit Eurostar to use German-made Siemens Velaro rolling stock, the French government dismissed their delegate to the CTSA, and brought in a replacement.
References
Channel Tunnel Safety Authority Wikipedia(Text) CC BY-SA