HMS Mersey was built by Vosper Thornycroft in Southampton, England to serve as a fishery protection vessel within the United Kingdom's waters along with her two sister ships Tyne and Severn.

[6] On 3 January 2019, Defence Secretary Gavin Williamson confirmed that HMS Mersey has been deployed to assist UK and French authorities with illegal migrant crossings in the English Channel.

[7] On 24 April 2017, in a written answer to a question raised by Sir Nicholas Soames, Parliamentary Under-Secretary for Defence Harriett Baldwin stated that Mersey would be decommissioned in 2019.

[8] In March 2018, Baldwin's successor Guto Bebb revealed that £12.7M had been allocated from the EU Exit Preparedness Fund to preserve the three Batch 1 ships, should they be needed to control and enforce UK waters and fisheries following the United Kingdom's withdrawal from the European Union.

[9] On 22 November 2018, Defence Secretary Gavin Williamson announced that three Batch 1 River-class ships would be retained in service and forward operated from their affiliated ports.