[5] Although the Nimrod option was identified as the cheapest, the cost of procuring and modifying three aircraft was still estimated at £14m, a significant proportion of the budget for SIGINT operations.

[4] This was owing to the highly secret nature of the equipment intended for use on the aircraft - instead, they were fitted out at RAF Wyton, the home base of 51 Squadron.

The first aircraft was delivered in July 1971, and was eventually completed more than two years later, making its first training flight in 21 October 1973, before being formally accepted into service in May 1974.

[10] The main peacetime use of the Nimrod was largely similar; the ability of the Nimrod to make a high speed transit to its operational area, and then loiter for an extended period,[1] meant that missions would usually involve sitting off the edge of the Soviet sphere of influence receiving and recording signals, which would subsequently be analysed by GCHQ.

[11] On 16 May 1995, on a test following a major service at RAF Kinloss, one of the three Nimrod R1s suffered a double engine fire and was forced to ditch in the Moray Firth.

The Nimrod R1 fleet, owing to its significantly reduced level of usage compared to the MR2, was originally intended to remain in service for an extended period into the 2010s, with a major systems upgrade codenamed Project HELIX.

[14] However, in October 2008, the UK Government made a request into the possibility of procuring new aircraft for the SIGINT mission, specifically the RC-135 Rivet Joint, under a new project codenamed Airseeker.

In 2009, with the plan for the Rivet Joint making progress, one of 51 Squadron's three Nimrods was withdrawn from service to be used as a spares source for the remaining two.