• The Ministry of Aircraft Production (MAP). MAP was responsible, on the advise of the Establishments, for the clearance of aircraft and related equipment.
• The Aeroplane and Armament Experimental Establishment (A&AEE) at Boscombe Down under MAP control. A&AEE’s primary role was that of testing aircraft to ensure they were in a fit state for their intended use by the Services. The Establishment was also concerned with aeronautical development and acceptance. Performance testing was carried out fundamentally to establish level speed, climb rate, range, and ceiling. Raw data from flight tests was corrected to standard conditions. Performance reduction methods accounted for position error, compressibility, temperature, and altitude. In addition to the basic performance tests, trials were carried out to assess cooling, handling, and cockpit contamination.
• The Royal Aircraft Establishment (RAE), Farnborough under the control of the Ministry of Aircraft Production. RAE was engaged in research and theoretical work as well as development of modifications and improvements. Farnborough's wind tunnel was used to test Spitfire aerodynamics and was particulary useful in developing improvements to the Spitfire's ailerons. Fit and finish were assessed and improvements recommended. Also noteworthy was RAE's analysis of captured aircraft.
• The Air Fighting Development Unit (AFDU) at Wittering and Duxford under the control of Fighter Command and the Air Ministry. The AFDU performed comparitive trials and developed and promulgated tactics effective against the enemy.
• The Department of Scientific and Industrial Research (DSIR) under the control of the Aeronautical Research Council. Involved in projects to analyse and improve performance.
• Rolls-Royce's flight test establishment at Hucknall. Flight testing of engines, conversions, and modifications.
• Vickers-Armstrong (Supermarine) at Worthy Down. Production and development testing.

An Air Ministry document from 29.10.43 details the protocol to use in arriving at performance figures for aircraft:

1.      There are 4 main stages in the evolution of performance for a new aircraft:-

2.      The limits within which the Stage (1) estimate can be guaranteed depend on whether the new aircraft is -

3.      With "Development" aircraft, original estimates should be accurate to within 1-2% in speed, and 50-100 ft./min. in rate of climb. With "New Type" aircraft, however, the error might be up to 6-7% in speed, and 200-250 ft./min. in climb.

4.      Flight Test figures, by themselves, should be treated with considerable reserve, since they may be obtained under non-standard conditions, and the aircraft flown may differ from production machine.

5.      From the prototype trials, Provisional Performance Curves can be obtained, modifying them, if necessary, to allow for the effect of the difference between the prototype and production machine.

6.      Only when trials with representative production machines have been carried out under known conditions can the Final figures be issued. These figures then represent the performance expected of an average production machine of this type.

7.      Any particular machine off the production line, however, may vary from the average in top speed and climb because of differences in engine power and general finish. The usual variation for single-engined fighters is up to 3% in top speed, and 150-200 ft./min. in rate of climb; heavy bombers vary up to 4% in top speed, and 150-200 ft./min. in climb. (...)

Jeffrey Quill, from his perspective as Chief Test Pilot for Supermarine, held the following view of Spitfire testing:

"Every major modification or change, especially those affecting performance or handling qualities, was exhaustively tested by the firm (Supermarine) before submission to the Aircraft and Armament Experimental Establishment at Boscombe Down for official approval. Establishing the accurate performance of any varient involved a lot of flying and a great deal of full power bashing of the engine and systems. As time went on Boscombe Down came increasingly to accept the firm's figures, making only spot checks themselves, in order to save wear and tear on the prototypes. In other words Boscombe Down provided the official seal of approval, though many of the performance figures quoted in their reports were in fact measured by Supermarine's experimental flight test unit at Worthy Down, later at Hight Point. The A and AEE made their own judgements on the aircraft's handling and other qualities. The Air Fighting Development Unit represented Fighter Command, and thus the main users of the Spitfire. They did a most useful job in relating the various British fighters to those of the enemy and in developing tactics on behalf of the command. They were fully entitled to express their opinions about the handling of aircraft and they certainly did so. But the people at Boscombe Down were the final arbiters of what was fit for service use and what was not, and whether or not and aircraft met its specifications and contractual conditions."