The authorised establishment of the various elements of the Army, together with actual strengths in brackets, on 1 August 1914 were:
Channel Islands & Militia
Territorial Force Reserve
Bermuda & Isle of Mann Reserve
Slightly less than half of the Regular Army was serving abroad, mostly in India.
The Army was divided into fighting and administrative services.
The Fighting Arms
Four Foot Guards and sixty-nine Line (i.e. 'County') Regiments totalling 157 Battalions.
The Royal Regiment of Artillery consisted of the Royal Horse Artillery (RHA) and the Royal Field Artillery (RFA); and the Royal Garrison Artillery (RGA), together commonly known as the Royal Artillery.
There were three Household Cavalry Regiments, there were also twenty-eight Line Regiments (Dragoons, Dragoon Guards, Hussars and Lancers). The cavalry regiment was a tactical unit (equivalent to an infantry battalion) and was divided into three squadrons.
The basic unit of the Engineers was the company of which three were sixty-six Regular and Special Reserve of various kinds at home on the outbreak of war and nineteen abroad. An infantry division contained field companies and a cavalry division field squadrons which were commanded by a Major and divided into four sections.
Royal Flying Corps (RFC):
The formation of the RFC had only commenced in 1912 and was still incomplete when the war started. Only four aeroplane squadrons and an aircraft park were therefore able to join the original BEF.
Upon mobilisation divisional cyclist companies were formed from infantry within division for reconnaissance duties and skirmishing.
The Machine Gun Corps (MGC) was created in October 1915 and consisted of three branches, Infantry, Cavalry and Motor.
Tanks were first used in September 1916 when four companies of the MGC (Heavy Section) went into action on the Somme.
Trench mortars were not strictly a separate arm as heavy mortars were manned by the RGA, medium by the RFA and light by Infantry.
Royal Defence Corps:
Consisted of older men or those not fit for overseas service.
The Administrative Services
Army Service Corps (ASC):
It was responsible for supplying food, forage and petrol to the Army, for road transport by horse or motor vehicles up to the point where units took over, remounts and the repair of motor vehicles.
Royal Army Medical Corps (RAMC):
The RAMC was responsible for sanitation, the health of the soldiers and the treatment of the sick and wounded. All officers, except a few on administrative duties, were medically qualified.
Army Veterinary Corps (AVC):
In 1914 an infantry division had 5,594 horses so the importance and scale of work of the AVC is apparent. Qualified Vet's held substantive ranks and there were 197 of them.
Army Chaplains Department:
There were no other ranks in this department and the Ordained chaplains held relative ranks from Captain to Colonel.
Queen Alexandria's Imperial Nursing Service:
This was the only women's service in the army and its ranks were Matron, Sister and Staff Nurse. In addition there were also Voluntary Aid Detachments (VADs) manned by voluntary aid societies.
Army Ordnance Corps (AOC):
With the exception of 'supplies' (see ASC) and certain technical stores, the AOC supplied everything that the armies in the field required including clothing, weapons, ammunition of all kinds and trench stores.
Army Pay Corps (APC):
The Officers belonged to the Army Pay Department, the Other Ranks to the Corps.
Corps of Military Police (CMP):
Consisted of two branches, the Military Mounted Police (MMP) and the Military Foot Police (MFP).
Military Provost Staff Corps (MPSC):
Staffed peacetime military prisons such as the (in)famous 'glasshouse' at Aldershot and the military prisons in the theatres of War.
Upon mobilisation an ad hoc Intelligence Corps for the BEF was formed from about fifty previously selected personnel.
Formed in April 1917 from men of lower medical category for unskilled work behind the lines.
Non-Combatant Corps (NCC):
Formed April 1916 from conscripted conscientious objectors with officers and NCOs from the infantry. They performed labouring duties.
Women's Army Auxiliary Corps (WAAC):
Formed March 1918 to release able bodied men for more active duty by undertaking clerical, catering, driving, storehouse and postal work.
More detail can be obtained by purchasing WFA Military Fact Sheet No.14
Martin Hornby - with acknowledgement to Bob Butcher.