UK — The UK's Crown Prosecution Service (CPS) successfully challenged an attempt by the US military to prosecute Airman Mikayla Hayes in a court martial, asserting British jurisdiction under the 1951 Status of Forces Agreement. Hayes, an American mechanic stationed at the Lakenheath airbase, was involved in a traffic collision in August 2022 that resulted in the death of British citizen Matthew Day.

The incident occurred when Hayes was driving from the Lakenheath airbase to her home in Downham Market, Norfolk, and struck a motorbike driven by Day, a 33-year-old Briton. The US military subsequently sought to assume control of the investigation, claiming that Norfolk police lacked the authority to arrest Hayes. A formal certificate was served on British authorities by the US to assert that Hayes should face a court martial.

The CPS launched a legal challenge to overturn this certificate, which proved successful. Following the quashing of the certificate, Hayes was prosecuted at Norwich Crown Court where she was acquitted of causing death by careless driving.

US military personnel in the UK are governed under the 1951 Status of Forces agreement, which outlines the terms of their operations. This agreement stipulates that the US government typically prosecutes its own military personnel if offenses are committed while on duty or against another member of the US armed forces, their property, or a dependent. In other cases, British police technically have the initial option to prosecute, although Americans can request that cases be relinquished to US jurisdiction. The British are required to give "sympathetic" consideration to such requests, but they retain the right to refuse.

A US air force expert in the US military justice system stated that the UK often transfers cases to the US military for prosecution. "Oftentimes the UK will turn it back over to the US military and allow us to prosecute it," the expert said. However, the expert added that the Americans would defer if the British insisted on prosecuting a case themselves. A US air force spokesperson further stated that the US air force's proceedings "are fair, transparent and thorough." The Pentagon maintains that prosecuting its own military personnel is a more effective method for maintaining discipline. "It's very important to our commanders to be able to have consistent, standardised punishments and systems for our troops," the expert said. He also noted that commanders "want to send a deterrent message."

There are over 12,000 US personnel stationed at a minimum of 15 US military bases and facilities across the UK, with the majority located in England. Trials for US military personnel typically occur on military premises, with judges, prosecutors, and defense lawyers who are legally trained and employed by the military. Juries in these trials consist of armed forces members. These court martial hearings are conducted within heavily guarded perimeters, and the public cannot freely access or observe the proceedings.