The British Trench Mortar Batteries in the First World War

Trench mortar team

This article is dedicated to the memory of the officers and men who served the Trench Mortars. Often the focus of infantry grumbling – for a front-line trench mortar was certain to draw enemy fire – the TM Batteries played an important part in gaining the ascendancy in both attack and defence.

Formation and development

In 1914, no organisation existed for trench mortars as the weapons simply did not exist in the British army. Along with the early and faltering development of the weapons, the organisation into batteries was rather haphazard and was left to local command. The infantry, artillery and engineers were all involved from time to time. By December 1915, high command ordered that light mortar units would be manned by the infantry and the medium/heavy units by the Royal Field Artillery. In March 1916, the medium trench mortar batteries formally came under the command of Divisional artillery, while the light Stokes batteries left battalion-level control and came under brigade command. At this time, each Division was given a Divisional Trench Mortar officer, the batteries were numbered, and a badge was introduced to identify mortar personnel.


In 1914, the Army was not equipped with trench mortars. The German Army had three types of Minenwerfer, although perhaps only as few as 160 in all. These weapons soon became a dreadful hazard for the scratch trenches in Flanders, the heavy weapon in particular firing a large canister bomb that could destroy many yards of trench. In response, the British authorities decided not to copy the German design on the basis of their inherently unsafe ammunition. Twelve experimental 3.7-inch mortars, with 545 rounds of ammunition, arrived in France in December 1914. They proved to be inaccurate,with a tendency to premature explosion. Forty ancient Coehorn mortars, firing spherical ammunition using black powder charges,were obtained from the French,and were actually fired at the battles at Neuve Chapelle and Aubers Ridge. They were nicknamed Toby mortars, after the officer whose initiative led to their acquisition. In desperation for a short-range artillery weapon, the infantry and engineering workshops improvised, making a variety of weapons,many more dangerous to the firer than to the target. Other devices built to achieve the same effect included catapults. During the first part of 1915, trench mortar production was pitifully small: 75 supplied in the first three months, then 225 in the second. However, the main bottleneck was in providing ammunition: only 8,816 rounds in the first Quarter, and 42,753 in the second. Various models including 1.57-inch, 2-inch, and 4-inch had joined the 3.7-inch in the poor fare with which the Army was supplied.

The breakthrough came in mid 1915, with the acceptance of the 3-inch Stokes mortar. This had been invented in January 1915 by Wilfred Stokes: a design of brilliant simplicity, which became standard issue in the Army for several decades. (200 4-inch Stokes projectors were also made, for gas-filled ammunition. 27 of these fired smoke mortars in the opening barrage at Loos in September 1915). The first production order for 1,000 weapons was issued in August, and 304 were issued in the final Quarter of 1915, of which 200 went to training schools. The Stokes mortar could be operated by skilled crews to have a very high rate of fire, with a number of rounds – perhaps up to nine – in flight at any one time.

By the time of the Battle of Loos in September 1915, the mortars had been arranged into 61 four-gun batteries. GHQ proposed to provide each Division with 6 light batteries, 2 medium and 1 heavy; but this had not been achieved even by the opening of the Battle of the Somme in July 1916. By May 1916 it was decided to standardise on three types: the 3-inch Stokes (‘light’), the 2-inch Medium (superceded in 1917 by the 6-inch Newton Mortar), and the 9.45-inch Heavy. The latter became available towards the end of 1916, after failed experiments in the summer. The army also called these ‘Flying Pigs’. By 1918 each Division had 24 Stokes and 12 Medium mortars, and a few 9.45-inch Heavy weapons.

Trench mortar tactics

Trench mortars were used in a variety of defensive and offensive roles, from the suppression of an enemy machine-gun, sniper post or other local feature, to the coordinated firing of barrages. Larger mortars were sometimes used for cutting barbed wire, especially where field artillery could not be used, either because of the danger of hitting British troops or where the effect of the fire could not be observed. Experience on the Somme revealed that use of Stokes mortars in an offensive close-support role had been limited by the reluctance of some commanders to sacrifice rifle strength to provide parties required to carry the ammunition which the weapons so quickly consumed.

Trench mortar units

By March 1916, most Divisions had three Medium Batteries, designated X, Y and Z. For example, in the 24th Division they would be X.24,Y.24 and Z.24. There was also the Heavy Battery, designated V, such as V.24. The light Stokes batteries under each Brigade took their number from the Brigade, so for example 123rd Brigade in the 41st Division included 123rd TM Battery from June 1916. Z Battery was in most cases broken up in February 1918, with personnel redeployed to the other batteries.

Organisation of a Trench Mortar Battery

Light Trench Mortar Battery (2 Sections, each of four 3-inch Stokes mortars)
Captain, 3 Lieutenants or Second Lieutenants, 2 Sergeants, 8 Corporals or Lance-Corporals, 32 Privates, 4 Batmen.
Medium Trench Mortar Battery (four 2-inch mortars)
2 Officers, 1 Sergeant, 4 Corporals or Bombardiers, 16 Privates, 2 Batmen.
Heavy Trench Mortar Battery (four 9.45-inch mortars)
Captain, 2 Lieutenants or Second Lieutenants, 3 Sergeants, 1 Fitter, 4 Corporals, 4 Bombardiers, 47 Gunners (including 3 telephonists), 2 Orderlies, 1 Clerk, 1 Cook, 3 Batmen.

Trench Mortar Schools

Specialist training of trench mortar officers and men was carried out at schools, organised one per Army in France.

First Army – located at Saint-Venant (and had originally been I Corps Trench Mortar School)

Second Army – at Berthen (certainly by spring 1915) but later Leulinghem (by mid-1917)

Third Army – at Saint-Pol-sur-Ternoise



The Royal Artillery