First, it should be reminded that the Bren LMG was a gas-operated weapon using the same .303 British Mk VII Cartridge as the standard British bolt-action rifle № 1, № 4, and № 5, firing at a rate between 480 and 540 rounds per minute (rpm), depending on the model.  There was one Bren LMG per infantry section, plus some for Anti-Aircraft duty and therefore an average of 30 per infantry battalion.  The Bren LMG was operated by a team of two soldiers: a gunner (number 1) and a loader (number 2) who carried in addition to Basic Pouches with two magazines each, a pair of Utility Pouches each containing three magazines, a machete, and a spare barrel.

The Bren LMG Mk I was officially commissioned into service with the British Army in August 1938.  It was also manufactured in Canada by John Inglis Co Ltd in 1938.  The British Expeditionary Force (BEF) of 1939 went to war equipped with their new Bren LMG, including one spare barrel per gun, one sustained-fire tripod for every three guns and a Fixed Line Sight (FLS).  

During the Battle of France, the Dunkirk evacuation (codenamed “Operation DYNAMO), saw between 26 May and 4 June 1940, the evacuation of more than 338,000 Allied soldiers from the beaches and port of Dunkirk, in northern France.  The British Expeditionary Force (BEF) suffered over 68,000 casualties in addition to abandoning all its equipment in France.  The Dunkirk debacle witnessed Britain losing more than 90% of its Bren LMG inventory.  Thus, of the 30,000 Bren LMGs that it had at the start of the Battle of France (10 May 1940), Britain only had some 2,300 Bren LMGs left.  This put Britain desperately short of light machine guns, and so it was decided to modify the Bren’s design to make it simpler and therefore faster to manufacture.  The need became even greater with Japan’s entry into the war in late 1941.  

In order to replace the Bren LMG losses suffered during the evacuation as well as to accelerate Bren LMG production, a “two-pronged approach” was adopted, and two schemes were approved.  The first scheme will make modifications to the existing Mk I pattern and will be known as pattern Mk I (Modified).  This will only require some tooling modifications and thus the Bren LMG will continue to be produced at Royal Small Arms Factory (RSAF) Enfield.  The second scheme will see a redesign of several parts in order to simplify the design and thus speed up its production.  This required new tooling.  

The Mk I (Modified) modifications involved elimination of some machining operations and simplifying others.  The modifications included the elimination of the dovetail slot for the ineffective Lensatic sight, the flutes surrounding the gas cylinder, the butt strap, and the butt handle.  This series of modifications resulted in a cost saving of 20% to 25% or around £10 on an average cost of £40 (equivalent in purchasing power to about £2,806.21 in 2024).  

The Mk II, introduced in 1941, was a simplified version of the Mk I (Modified).  More suited to wartime production with original design features subsequently found to be unnecessary deleted.  Among the changes were a new folding leaf backsight instead of a drum one, simplified bolt, less body machining, fixed (non-folding) cocking handle, and fixed height bipod.  In Britain, the Mk II was produced by the consortium “Monotype Group” through several component manufacturing factories (eight sub-contractors).  Production in Canada started in 1940 at the Inglis plant and by August 1942 the production was averaging 10,000 Bren LMGs per month.  By 1943 some 60% of Bren LMG production was eventually carried out in Canada.  

A chronic shortage of the weapon persisted until late 1942, when production of the gun by the UK, Canada, and Australia made up the shortfall.  By the end of the war, Bren LMG Mk I, Mk I (Modified) and Mk II were in significant numbers in all British fighting divisions: 1,262 per armored division, 1,376 per infantry division, and 966 in one airborne division.