Nine Boeing Poseidon MRA1 were ordered by the Government in November 2015 in its Strategic Defence and Security Review for surveillance, anti-submarine and anti-surface ship warfare, filling a capability gap in maritime patrol that had been left since the cancellation of the BAE Systems Nimrod MRA4 programme in the 2010 SDSR. On 13 July 2017, it was announced that No. 120 Squadron and No. 201 Squadron, both former Nimrod MR2 squadrons, would operate the Poseidon and be based at RAF Lossiemouth. No. 120 Squadron stood up on 1 April 2018, with No. 201 Squadron reforming on 7 August 2021. No. 54 Squadron acts as the OCU for the Poseidon fleet.
The first production Poseidon MRA1 ZP801 made its initial flight on 13 July 2019. ZP801 arrived at Kinloss Barracks, the former home of the Nimrod, on 4 February 2020, filling a decade long gap in maritime capability. The Poseidon was declared combat ready in April 2020. The Poseidon carried out its first operational mission on 3 August 2020, when the Russian warship Vasily Bykov was tracked. A Poseidon MRA1 arrived at RAF Lossiemouth for the first time in October 2020. The ninth, and final Poseidon arrived at RAF Lossiemouth on 11 January 2022.