- On the outbreak of the First World War (WW1), The London Regiment was embodied and all Battalions raised 2nd line Battalions. Later, 3rd line Battalions were raised; and the 1st , 2nd , 3rd and 4th Battalions, The London Regiment raised 4th line Battalions. By the end of 1914, the original 26 Battalions had expanded to 58 Battalions; and by May 1915 they had reached their maximum strength of 82 Battalions.
- In April 1916, the 4th Battalions of the 1st , 2nd , 3rd and 4th , and the 3rd Battalions of the 5th to 25th , were redesignated Reserve Battalions. Shortly afterwards, the 2nd Battalions of the 1st , 2nd , 3rd and 4th were absorbed into their 1st Battalions, and their 3rd Battalions were re-numbered as 2nd Battalions. Thus in June 1916 each of the original 1st to 25th Battalions consisted of a 1st line Battalion, a 2nd line Battalion and a Reserve Battalion; e.g. 1st / 23rd , 2nd / 23rd , and 23rd (Reserve). The 28th now had only one Battalion – the 2nd / 28th had been absorbed into the 1st / 28th , and the 3rd / 28th had become 15th Cadet Battalion Officer Training Corps. Thus in June 1916, the London Regiment consisted of 76 Battalions.
- In July 1916, all Battalions were posted back to regular Regiments. Thus The London Regiment, as a separate Regiment, lasted only just over eight years. The name, however, continued, as Battalions did not change their titles.
- In most cases, the regular Regiment to which Battalions of The London Regiment were now posted were those of which, prior to 1908, they had been Volunteer Battalions. However there were some changes.
- Three Battalions of The London Regiment which before 1908 had been Volunteer Battalions of The King's Royal Rifle Corps were now part of a different regular Regiment. They were:
5which joined The Rifle Brigade (Prince Consort's Own)7, 13which joined The Duke of Cambridge's Own (Middlesex Regiment)
- Three Battalions of The London Regiment which before 1908 had been Volunteer Battalions of The Rifle Brigade (Prince Consort's Own) were now part of a different regular Regiment. They were:
14which joined The Gordon Highlanders18which joined the Royal Irish Rifles28which became an Officer Training Corps
- In July 1916, The London Regiment stood as follows:
The Queen's (Royal West Surrey Regiment):
1st / 22nd , 2nd / 22nd and 22nd (Reserve) (County of London) Battalions,
The London Regiment (The Queen's)
1st / 24th , 2nd / 24th and 24th (Reserve) (County of London) Battalions,
The London Regiment (The Queen's)The Royal Fusiliers (City of London Regiment):
1st / 1st , 2nd / 1st and 1st (Reserve) (City of London) Battalions,
The London Regiment (Royal Fusiliers)
1st / 2nd , 2nd / 2nd and 2nd (Reserve) (City of London) Battalions,
The London Regiment (Royal Fusiliers)
1st / 3rd , 2nd / 3rd and 3rd (Reserve) (City of London) Battalions,
The London Regiment (Royal Fusiliers)
1st / 4th , 2nd / 4th and 4th (Reserve) (City of London) Battalions,
The London Regiment (Royal Fusiliers)The East Surrey Regiment:
1st / 21st , 2nd / 21st and 21st (Reserve) (County of London) Battalions,
The London Regiment (First Surrey Rifles)
1st / 23rd , 2nd / 23rd and 23rd (Reserve) (County of London) Battalions,
The London RegimentThe Queen's Own (Royal West Kent Regiment):
1st / 20th , 2nd / 20th and 20th (Reserve) (County of London) Battalions,
The London Regiment (Blackheath and Woolwich)The Duke of Cambridge's Own (Middlesex Regiment):
1st / 7th , 2nd / 7th and 7th (Reserve) (City of London) Battalions,
The London Regiment
1st / 13th , 2nd / 13th and 13th (Reserve) (County of London) Battalions,
The London Regiment (Princess Louise's Kensington)
1st / 19th , 2nd / 19th and 19th (Reserve) (County of London) Battalions,
The London Regiment (St Pancras)The King's Royal Rifle Corps:
1st / 6th , 2nd / 6th and 6th (Reserve) (City of London) Battalions,
The London Regiment (City of London Rifles)
1st / 9th , 2nd / 9th and 9th (Reserve) (County of London) Battalions,
The London Regiment (Queen Victoria's)
1st / 11th , 2nd / 11th and 11th (Reserve) (County of London) Battalions,
The London Regiment (Finsbury Rifles)
1st / 12th , 2nd / 12th and 12th (Reserve) (County of London) Battalions,
The London Regiment (The Rangers)
1st / 15th , 2nd / 15th and 15th (Reserve) (County of London) Battalions,
The London Regiment (Prince of Wales's Own, Civil Service Rifles)
1st / 16th , 2nd / 16th and 16th (Reserve) (County of London) Battalions,
The London Regiment (Queen's Westminster Rifles)The Gordon Highlanders:
1st / 14th , 2nd / 14th and 14th (Reserve) (County of London) Battalions,
The London Regiment (London Scottish)The Royal Irish Rifles:
1st / 18th , 2nd / 18th and 18th (Reserve) (County of London) Battalions,
The London Regiment (London Irish Rifles)The Rifle Brigade (Prince Consort's Own):
1st / 5th , 2nd / 5th and 5th (Reserve) (City of London) Battalions,
The London Regiment (London Rifle Brigade)
1st / 8th , 2nd / 8th and 8th (Reserve) (City of London) Battalions,
The London Regiment (Post Office Rifles)
1st / 10th , 2nd / 10th and 10th (Reserve) (County of London) Battalions,
The London Regiment (Hackney)
1st / 17th , 2nd / 17th and 17th (Reserve) (County of London) Battalions,
The London Regiment (Poplar and Stepney Rifles)
1st / 25th , 2nd / 25th and 25th (Reserve) (County of London) Cyclist Battalions,
The London RegimentOfficers' Training Corps:
28th (County of London) Battalion, London Regiment (Artists Rifles)
Expansion in The Great War and Re-linkage with Regular Regiments