fifth column 
noun
A group within a country at war who are sympathetic to or working for its enemies.While thus ‘raising public awareness,’ Congress was being lobbied for legislation to confront the threat from this enemy within: the fifth column in the ivory tower....
- Democratic governments played on the fear of a fifth column within in order to bolster the national purpose.
- The creation of a potential fifth column within the organization makes the duties of members of the audit committee far more onerous than before, with no relief in sight.
Derivatives
fifth columnist
/ noun ...- Insurrectionists, traitors and fifth columnists were a feature of our political landscape.
- One of the main themes is the threat posed by fifth columnists, and Fred, despite his admirable single-mindedness, is dangerously naïve in his dealings with strangers.
- Does Ms. West not think such fifth columnists exist?
Origin
The term dates from the Spanish Civil War, when General Mola, leading four columns of troops towards Madrid, declared that he had a fifth column inside the city.