The Auxiliary Units or GHQ Auxiliary Units were specially-trained, highly-secret quasi military units created by the British government during the Second World War with the aim of using irregular warfare in response to a possible invasion of the United Kingdom by Nazi Germany, "Operation Sea Lion". With the advantage of having witnessed the rapid fall of several Continental European nations, the United Kingdom was the only country during the war that was able to create a multilayered guerrilla force in anticipation of an invasion.

The Auxiliary Units would fight as uniformed guerrillas during the military campaign. In the event of an invasion, all Auxiliary Units would disappear into their operational bases and would not maintain contact with local Home Guard commanders, who were to be wholly unaware of their existence. Although the Auxiliaries were Home Guard volunteers and wore Home Guard uniforms, they would not participate in the conventional phase of their town's defence but would be activated once the local Home Guard defence had been ended to inflict maximum mayhem and disruption over a further brief but violent period. They were not envisaged as a continuing resistance force against long-term occupation. The secrecy surrounding the insurgent squads meant that members “had no military status, no uniforms and there are very few official records of their activities”.

Service in the Auxiliary Units was expected to be highly dangerous, with a projected life expectancy of just twelve days for its members, with orders to either shoot one another or use explosives to kill themselves if capture by an enemy force seemed likely.

Urged on by the War Office, Prime Minister Winston Churchill initiated the Auxiliary Units in the early summer of 1940. This was to counter the civilian Home Defence Scheme already established by SIS (MI6), but outside War Office control. The Auxiliary Units answered to GHQ Home Forces but were legally an integral part of the Home Guard.

In modern times, the Auxiliary Units have sometimes misleadingly been referred to as the "British Resistance Organisation". That is a title was never used by the organization officially but reflects a subsequent misunderstanding of what their role might have been. Colloquially, members of the Auxiliary Units were referred to as "scallywags" and their activities as "scallywagging".