21 Anti-Tank Regiment RA was formed in November 1938 by conversion of 21 Field Brigade RA with four batteries. In November 1946, RHQ and three batteries became 4 RHA. The identity of 21 Anti-Tank Regiment RA passed to 21 Field Regiment. In June 1947, RHQ 21 Field Regiment re-organised as RHQ 4 RHA and the identity of 4 RHA passed to RHQ 10 Anti-Tank Regiment.
Illustrations of the formation badges under which a unit served or supported are given. This is no guarantee that the unit used this formations badge.
| 3 Sep 39 |  |
1 Infantry Division | Aldershot, Hampshire, UK |
| Sep 39 | |
1 Infantry Division | France |
| Jun 40 | |
1 Infantry Division | UK |
| Nov 40 | Cadre to form 74 Anti-Tank Regiment | ||
| Oct 41 |  |
Guards Support Group | UK |
| Jun 42 | |
Guards Armoured Division | UK |
| Jun 44 | |
Guards Armoured Division | NW Europe |
| Jun 45 |  |
7 Armoured Division | NW Europe |
Batteries
| 3 Sep 39 | Q, Y, Z, BB |
| Sep 41 | BB Battery to form 81 Anti-Tank Regiment |
| Jul 45 | Q (Sanna’s Post), Y, Z, 2 (Minden) |