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An unprecedented look inside MI6 - Britain's Secret Intelligence Service, which marked its centenary in 2009.

BBC security correspondent Gordon Corera talks to senior intelligence officers, agents and diplomats as well as their former arch enemies about the shadowy world of espionage.

Part One - Gadgets & Green Ink

The first part of this series explores the early years of MI6 - set up by Sir Mansfield Cumming, an eccentric and formidable figure known as 'C' who signed his name in green ink.

The highs and lows of the two world wars and details of some of the gadgets any self respecting agent could not do without.

This is an updated version of the documentary that was first broadcast on the BBC World Service on 12 October 2009.

Originally broadcast on BBC Radio 4.