The raising of four new Siege Batteries of the Royal Garrison Artillery was authorised by Army Council Instruction 1091/1916, with effect from 22 March 1916.
In addition, Army Council Instruction 777/1916 authorised the raising of a South African battery from 3 April 1916.
| RGA Siege Battery | Location |
|---|---|
| 121 | Pembroke Dock |
| 122 | Dover |
| 123 | Portland |
| 124 | Portsmouth |
| 125 (South African) | Bexhill |
The ACI stated that the establishment of each battery would be that laid down for batteries equipped with 6, 8 or 9.2-inch howitzers.
Three officers and 78 men from the Glamorgan, Sussex and Dorset RGA (Territorial Force) were to be allotted to 121, 122 and 123 Siege Batteries RGA respectively. The South African battery would be raised from men who were at the South African Heavy Artillery depot at Bexhill in Sussex.