UK intelligence officials said to be providing information on targets for CIA drone strikes in Pakistan could be "encouraging or assisting murder" and the commission of war crimes, the High Court heard.
The son of a Pakistani man killed by a missile from a drone in the north-west tribal area wants a declaration that the UK Government's alleged policy of passing on information on the location of individuals to the CIA is unlawful.
Lawyers for Foreign Secretary William Hague are urging two judges to block the application brought by 27-year-old Noor Khan.
They argue that the case raises issues relating to sovereign foreign states that cannot be determined by the English courts.
They also say that adjudicating on Mr Khan's claim would plainly have a "significant impact" on the conduct of the UK's relations with the US and Pakistan in an "acutely controversial, sensitive and important" context, and also impact on relations between the US and Pakistan.
Lord Justice Moses and Mr Justice Simon are deciding over two days at the High Court in London whether the application should go to a full hearing.