Rookie wartime MI5 "watchers" were advised they needed to be prepared to offer a big tip if they had to tell a taxi driver to "follow that cab" when pursuing a suspect, newly declassified documents reveal.

The manual gives instructions on preferred appearances, disguises and how to follow targets – but warns against fake moustaches

The documents, on display in the National Archives in Kew, West London, suggest the organisation was keen to dismiss any fantasies about the glamour of espionage.

The guidance will feature in a new exhibition at the National Archives focusing on the work of the Security Service.