Dubbed the DAA, because the actual title Access to Electronic Data for the Purpose of Countering Serious Crime is longer than War and Peace, is intended to facilitate cross-border law enforcement within the boundaries set by privacy and civil liberties laws.

US Justice Department said: "Under the Data Access Agreement, service providers in one country may respond to qualifying, lawful orders for electronic data issued by the other country, without fear of running afoul of restrictions on cross-border disclosures. The Data Access Agreement fosters more timely and efficient access to electronic data required in fast-moving investigations through the use of orders covered by the Agreement."

The Justice Department says the DAA will facilitate the prevention, detection, investigation, and prosecution of serious crime, such as terrorism, transnational organised crime, and child exploitation. The UK Home Office said that it will help the UK particularly because so much online data is held by companies operating with the US where it hasn't been easily available.

"Many of the currently popular telecommunications services, such as social media platforms and messaging services, operate within US jurisdiction," the Home Office said in a recent policy paper.

"Unfortunately, US law prohibits these companies from being able to share certain data in response to a request made directly by a foreign government. This means that data which might be essential to an investigation cannot be obtained."

The bulk of legal demands for data are expected to be served on US communications service providers by UK authorities because there are comparatively fewer UK companies holding data of interest to US law enforcement.