Descended from the Westminster Trained Bands, which were on duty during the Armada campaign of 1588 and saw considerable service during the English Civil War, the regiment underwent reorganisation in 1760 as part of the Middlesex Militia.

The universal obligation to military service in the Shire levy was long established in England and its legal basis was updated by two acts of 1557 (4 & 5 Ph.

Although the militia obligation was universal, it was clearly impractical to train and equip every able-bodied man, so after 1572 the practice was to select only a proportion of men for the, who were mustered for regular drills.

The two Westminster regiments saw action during the cvil war at the Siege of Basing House and the battles of Alton, Cropredy Bidge and Second Newbury.

Although most of the militia declined after the Peace of Utrecht in 1713, the London Trained Bands remained fully active, and the Westminster units continued to appear at least as late as 1728 (the colonel in 1722 was Robert Gardiner).

An adjutant and drill sergeants were to be provided to each regiment from the Regular Army, and arms and accoutrements would be supplied when the county had secured 60 per cent of its quota of recruits.

However, opinion in the county shifted and in July 1760, the lieutenancy began forming three regiments (Eastern, Western and Westminster) and the arms and accoutrements were supplied from the Tower of London on 7 and 12 August However, by then the war was going in Britain's favour and the threat of invasion had lifted: no further militia were required, and the Middlesex regiments were not actually embodied before the war ended in 1762.

[26] From 1784 to 1792 the militia were assembled for their 28 days' annual peacetime training, but to save money only two-thirds of the men were actually mustered each year.

The French Revolutionary Wars saw a new phase for the English militia: they were embodied for a whole generation, and became regiments of full-time professional soldiers (though restricted to service in the British Isles), which the regular army increasingly saw as a prime source of recruits.

The colonel, John Fenton-Cawthorne, Member of Parliament, was later accused of withholding the 'Marching Guineas' he had received from the paymaster to the forces – the sum of 21 shillings to be paid through the company commanders to every militiaman on embodiment for his clothing and kit other than the basic uniform.

The men complained again in September, and by November Gen Sir William Howe and Lt-Gen Lascelles again ordered Cawthorne to pay the Marching Guineas, without effect.

While in camp near Hythe in the summer of 1795 he was found to be submitting 'false musters' (including on the roll the names of men who were not in the regiment, in order to pocket their pay).

After a long and widely reported trial, the court found him guilty on seven of the 14 charges laid against him, and ordered him to be cashiered from the service.

[26][29][30] During the summer of 1805, when Napoleon was massing his 'Army of England' at Boulogne for a projected invasion, the Westminsters, with 883 men in 10 companies under Lt-Col William Allen, were stationed at Norman Cross Barracks as part of a brigade under Brigadier-General Nicholas Nepean.

Although officers continued to be commissioned into the militia and ballots were still held during the long peace after the Battle of Waterloo, the regiments were rarely assembled for training and the permanent staffs of sergeants and drummers were progressively reduced.

[27][55][56] War having broken out with Russia in 1854 and an expeditionary force sent to the Crimea, the militia began to be called out for home defence.

The Royal Westminsters were embodied on 6 February 1855 and the regiment was initially stationed at Turnham Green,[29][57][26][30] moving to Plymouth by beginning of July[58] It then volunteered for garrison duty overseas and was sent to Corfu by October.

The Militia Reserve introduced in 1867 consisted of present and former militiamen who undertook to serve overseas in case of war.

[55][63][64][65][66] Although often referred to as brigades, the sub-districts were purely administrative organisations, but in a continuation of the Cardwell Reforms a mobilisation scheme began to appear in the Army List from December 1875.

Between 1877 and 1880 the Royal Westminsters moved their HQ from Turnham Green to the brigade depot at Hounslow, joining the other Regular and Militia battalions.

At the same time the Royal Fusiliers took the subtitle 'City of London Regiment'[29][26][30][27][66][67][68] Militia battalions now had a large cadre of permanent staff (about 30).

[29][26][30][72] During the late 1890s several regiments recruiting from large conurbations, including the Royal Fusiliers in London and its suburbs, were increased from two to four regular battalions.

[26][27][30] After the disasters of Black Week at the start of the Second Boer War in December 1899, most of the Regular Army was sent to South Africa, and many militia units were embodied to replace them for home defence and to garrison certain overseas stations.

It was quartered near Kroonstad in the Orange Free State with detachments manning the blockhouses for about 8 miles (13 km) of the railway running north from the town, and some of the local defences.

On 21 August the battalion was moved about 30 miles (48 km) south to Ventersburg Road Station where it took over all blockhouses from Bloomspruit Bridge to a mile south of Ventersburg.On 7 January 1902 a detachment went to do duty at Eengevonden, and on 16 February a larger detachment occupied the blockhouses near the Vet River Station.

The war was ended by the Treaty of Vereeniging on 31 May, and on 19 June Battalion HQ entrained for Cape Town, picking up the Eengevonden and Vet River Station detachments on the way.

There were moves to reform the Auxiliary Forces (Militia, Yeomanry and Volunteers) to take their place in the six Army Corps proposed by the Secretary of State for War, St John Brodrick.

[26][30] On the outbreak of World War I the 5th Royal Fusiliers mobilised on 4 August 1914 at Hounslow under Lt-Col Vivian Henry, a retired regular major, who had commanded the battalion since 4 May 1908.

From 1855 to 1881 the officers' waist-belt plate carried the combined shield surmounted by the bugle horn, within a circle inscribed '3rd or Royal Westminster Militia'.

In that year the King drew the lots for individual regiments and the resulting list remained in force with minor amendments until the end of the militia.