It has a NATO ranking code of OF-3, equivalent to a Lieutenant Commander in the Royal Navy or a Major in the British Army.
On 1 April 1918, the newly created RAF adopted its officer rank titles from the British Army, with Royal Naval Air Service Lieutenant-Commanders and Royal Flying Corps Majors becoming Majors in the RAF. In response to the proposal that the RAF should use its own rank titles, it was suggested that the RAF might use the Royal Navy's officer ranks, with the word "Air" inserted before the naval rank title. For example, the rank that later became Squadron Leader would have been Air Lieutenant-Commander. However, the Admiralty objected to this modification of their rank titles. The rank title Squadron Leader was chosen as Squadrons were typically led by RAF Majors and the term Squadron Commander had been used in the Royal Naval Air Service. The rank of Squadron Leader has been used continuously since 1 August 1919.
Royal Air Force
Before the Second World War, a Squadron Leader commanded a squadron of aircraft. Today, however, a flying squadron is usually commanded by a Wing Commander, with each of the two flights under a Squadron Leader. However, squadrons which are administrative sub-divisions of a Wing are ordinarily commanded by a Squadron Leader.
Squadrons of the RAF Regiment are normally commanded by a Squadron Leader. Today the rank Squadron Leader is seen as the first Senior Officer rank within the RAF.
Air Training Corps
Within the Air Training Corps a squadron will normally be commanded by a Flight Lieutenant unless it is a particularily large squadron. Instead the Squadron Leader rank is more common amongst Wing Staff Officers.