An MI5 agent pretending to have Gestapo links secretly controlled a vast network of UK-based Nazi sympathisers, newly-released files reveal.

The agent, known by the alias Jack King, infiltrated a group of pro-German activists in south-east England in a previously unknown wartime operation.

His undercover work led to the identification of hundreds of people willing to pass secrets to the enemy.

Files in the National Archives describe King as a "genius" at his work.

In 1942, King - also referred to as SR in the files - met Marita Perigoe, described as a "crafty and dangerous woman" of Swedish and German origin....