A police force has grounded a £40,000 "eye in the sky" amid concerns it was being used illegally. The remote control drone helicopters, fitted with CCTV and thermal imaging cameras, are used by several forces. Last week Merseyside Police announced that the military-designed device had been involved in its first arrest when it helped officers track down a suspected car thief in thick fog. But now the Civil Aviation Authority (CAA) has said it never gave Merseyside the required permission to use the drone for surveillance purposes.

A CAA spokesman said: "The CAA's Aviation Regulation and Enforcement Department is currently investigating the use of Unmanned Aerial Systems (UAS) by Merseyside Police. Merseyside Police has confirmed to the CAA that it has stopped all UAS operations.

"The CAA has been in consultation with police forces to ensure UAS operations are introduced and conducted in accordance with UK aviation regulations."

Merseyside Police, who took delivery of the hi-tech device last year, said it can be used for a range of purposes, from serious firearms incidents and hostage situations to monitoring large public events and football matches. The drone has a top speed of 30mph with a ceiling of 400ft and comes with flashing police lights.

A spokesman for the force said: "Over the past year Merseyside Police has engaged with the CAA, as part of the UAS working group, to ensure that the force's use of its vehicle complies with current and anticipated CAA regulations.

"The CAA introduced new statutory regulations on 1 January 2010 which requires all UAS to be licensed by the CAA before use. Since the force has known of the change in regulations, all flights have been suspended.

"The force has written to the CAA to invite them to visit and inspect the vehicle and operating procedures."