Following the end of World War II, the British had a great many escort ships in service, ranging from the small "Flower" class corvettes built early in the war to the newly-constructed "Castle," "Loch" and "Bay" classes. However, the introduction of the fast German U-Boats of the Type XXI, Type XXIII and Type XXVI classes and the expectation that new Soviet submarines would have comparable performance had rendered all these escort vessels obsolescent almost overnight. As a result, a great many new sloop, frigate and other escort designs were started in the 1945-1950 period to meet this perceived threat, all with a confusing array of capabilities and armaments along with the almost obligatory blurring of distinctions between the different designations.

In an effort to reduce confusion, the Admiralty decided that single purpose escorts were now to be known as "Frigates" and accordingly introduced a system of two-digit Type numbers to define each design, with the first digit identifying the purpose and the second digit identifying the class. This system came into use starting in July 1950. The following three Frigate categories were established at that time:

1. Type 1X were Anti-Submarine (ASW) Frigates (when the numbers ran out in the 1960s, ASW Frigates continued as the Type 2X series and later as the Type 3X series)
2. Type 4X were Anti-Aircraft (AAW) Frigates (this later evolved into the "Destroyer" Type series)
3. Type 6X were Aircraft-Direction (ADW) Frigates

In addition to the single-purpose Frigates, the Admiralty further defined multi-role escorts as "destroyers" if they could achieve fleet speed or as "sloops" if they could not. These were to be numbered in the Type 8X series.

Designation	Notes
Type 11	Not officially assigned, but possibly an early designation for what became the Type 14
Type 12	"First Rate" vessels of the Whitby and Rothesay classes. Additional ships of this Type were built for India and South Africa. Four ships of this Type were built in Australia for the RAN.
Improved Type 12	Leander class
Type 13	Not assigned
Type 14	"Second Rate" vessels of the Blackwood class
Type 15	Conversion of "Emergency" destroyers of the "R" class
Type 16	Conversion of "Emergency" destroyers of the "T", "O" and "P" classes
Type 17	"Third Rate" vessels. Envisioned as a "cheap" ASW that could be mass-produced, this design was abandoned in 1953.
Type 18	Conversion of destroyers of the "N", "S", "T" and "Z" classes intended to replace the Type 16 (which was considered to be unsatisfactory), this design was abandoned in 1953.
Type 19	Intended as another cheap frigate, this concept was abandoned in 1965
Type 20	Not assigned
Type 21	Amazon class
Type 22	Broadsword class
Type 23	Duke class
Type 24	"Future Light Frigate," this was another attempt at a cheap design intended mainly as an export item. No sales resulted and hence none were built.
Type 25	A more capable version of the Type 24, this design was also not pursued.
Type 26	City class. Formerly known Future Surface Combatant program, also known as Global Combat Ships. Scheduled to enter service by early 2020s.
Type 27	Planned improved Type 26 with advanced ASW capabilities. Not pursued.
Types 28, 29 and 30	Not assigned.
Type 31	Inspiration class. Low-cost Frigate formerly known as the Type 31e frigate or General Purpose Frigate (GPF) and intended to replace the Type 23 frigates.
Type 32	General purpose frigate.

01 October 2013 - Benchmark
10 October 2014 - Added Type 26 and Type 27
03 February 2022 - Added Type 31 and Type 32