Converted to an air defence role before World War I, it served as a searchlight unit during the Battle of Britain, the Blitz, then, as a light anti-aircraft gun unit, it served in the most heavily attacked part of the South Coast of England throughout 1942–44, including the V-1 flying bomb campaign (Operation Diver).

In 1886, the War Office (WO) began organising units of 'submarine miners' in the Volunteer Force to man the fixed minefields being installed to defend British seaports.

During the Battle of Cambrai in November 1917, III Corps sent 574th up to repair the canal bridge at Masnières but it found 29th Division's engineers already doing the work.

[16][17] The unit was reformed in the renamed Territorial Army (TA) in 1920, consisting of a single company based at the RE Barracks in Falmouth.

[22] During the 1930s, the threat of aerial bombing was taken seriously, and large numbers of dedicated AA searchlight units were created in the TA.

[20][21] As the international situation deteriorated, the TA was ordered to mobilise on 22 August 1939, and the company draw its equipment from store and began to take up its war stations.

The following month, the opening of the Battle of Britain saw the start of day and night air raids on Plymouth and the searchlights were frequently in action.

[23] Anti-Aircraft Command was expanding rapidly to meet the threat, and new intakes of recruits arrived in Plymouth to form the 509th S/L Battery on 15 September.

[25] In April 1941, the regiment was redeployed from Cornwall to North Devon, with RHQ at the Glenhaven Hotel in Mortehoe, and on 5 May was joined by the newly formed 545 and 555 S/L Btys from 230th and 231st S/L Training Regiments at Blandford Camp, but no equipment arrived for them until July and communications to the remote S/L sites were tricky.

[33][34] 81st S/L Regiment moved its HQ to Buckland Filleigh and spent the autumn testing its sites for their suitability to mount SLC radar.

[25] Further adjustments came in November, when 555 Bty deployed lights to defend RAF St Eval, while 509 and 545 Btys moved to Westward Ho!

[25][35][36] On 12 January 1942, Regimental HQ moved to Boxted House near Colchester and three of the batteries took over vacant LAA gun positions at the RAF airfields at Debden, Martlesham Heath, North Weald and Wattisham to learn the basics of LAA work before attending a training regiment.

Three days later, the regiment was deployed under 71st AA Bde as follows:[41][45] The gunners soon found that it was difficult to traverse their guns quickly enough to engage the fast-moving Focke-Wulf Fw 190 fighter-bombers coming in low over the coast, often in poor visibility, to drop their bombs or fire their cannons on the towns.

[41] Raids continued at intervals during the winter, with the gun sites reporting a few successes, and several near misses from bombs.

[41][46][47][48] Peak strength in LAA defences against the hit-and-run attacks was reached in March 1943, and the success rate began to rise that month.

On 11 March a force of 20 aircraft attacked Hastings, of which four were shot down (Category I hits) and one damaged (Cat III) by the massed LAA defences.

But on 3 April, when 12 enemy aircraft bombed and machine-gunned Eastbourne from the landward side, the Bofors gunners' predictors and range-finders were useless.

The Bofors guns were equipped with auto-loading devices to increase their rate of fire, and gunshields to protect the crews.

[46][50] Between 21 January and 14 March 1944, the Luftwaffe crossed SE England to carry out 11 night raids on London in the so-called Baby Blitz.

The Diver belt was therefore redeployed with emphasis on HAA and LAA guns firing out to sea as the V-1s approached land.

[54] All these units were heavily engaged until the autumn, when 21st Army Group overran the V-1 launching sites in northern France.

[56] By now, 21st Army Group fighting in North West Europe was suffering a severe manpower shortage, and at the end of January the regiment had to send 64 NCOs and men to infantry training regiments, receiving 64 infantrymen of lower medical category in exchange.

V-1 attacks declined after January, but the regiment witnessed continuous V2 activity during February, and there was a final flare-up of long-range V-1s launched from North Holland in March.

[56] After VE Day, the state of readiness was maintained on the regiment's gun positions until ordered to stand down by 9 AA Group on 20 May.

432 Bty (the original Cornwall and Devon Fortress RE) became an independent battery in 28 (Thames & Medway) AA Bde, while the rest of 131 LAA Rgt moved from Tilbury to Birmingham.

433 and 434 Btys began to disband in August, and RHQ 131 LAA Regiment officially ceased to exist at West Derby, Liverpool, on 7 March 1946.