The Merchant Navy is the maritime register of the United Kingdom

United Kingdom

The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...

, and describes the seagoing commercial interests of UK-registered ships and their crews. Merchant Navy vessels fly the Red Ensign

Red Ensign

The Red Ensign or "Red Duster" is a flag that originated in the early 17th century as a British ensign flown by the Royal Navy and later specifically by British merchantmen. The precise date of its first appearance is not known, but surviving receipts indicate that the Navy was paying to have such...

 and are regulated by the Maritime and Coastguard Agency (MCA)

Maritime and Coastguard Agency

The Maritime and Coastguard Agency is a UK executive agency working to prevent the loss of lives at sea and is responsible for implementing British and International maritime law and safety policy.This involves coordinating search and rescue at sea through Her Majesty's Coastguard , ensuring that...

. King George V bestowed the title of "Merchant Navy" on the British merchant shipping fleets following their service in the First World War; a number of other nations have since adopted the title.
The Merchant Navy has been in existence for a significant period in British history, owing much of its growth to British imperial expansion.