The 84th (Middlesex, London Transport) Heavy Anti-Aircraft Regiment, Royal Artillery was an air defence unit of Britain's Territorial Army raised from employees of the London Passenger Transport Board during the period of international tension that preceded the outbreak of World War II.

Following the Munich Crisis of 1938, the British War Office rapidly increased the size of Britain's anti-aircraft (AA) defences manned by part-time members of the Territorial Army (TA).

[2] In April 1940, 260th Bty was detached and deployed to Åndalsnes in Norway as part of Sickleforce, whose commander, Maj-Gen Bernard Paget, had urgently requested additional AA defences against daily air attacks.

[22] By the culmination of the campaign, in May, 84th HAA Rgt had been transferred to 52nd AA Brigade and was 'on its wheels ready to move to Tunis and Bizerta, immediately behind the leading battle groups' of Operation Vulcan.

When the Axis forces surrendered, it was in the process of deploying in Bizerta, which was designated an 'Inner Artillery Zone' for air defence.

[25][26] Forward deployment could be dangerous: on 29 August a Troop of 84th HAA south of the River Arno was heavily shelled for an hour and suffered casualties and loss of equipment, but no guns were put out of action.

The result was that a number of AA units were deemed surplus and were disbanded to provide reinforcements to other arms of service.

84th HAA Regiment was the unit selected from 2 AA Bde, and it was broken up in October and formally placed in 'suspended animation' on 7 November 1944.