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

Despite advances in electronic surveillance, security service MI5 still relies on human ‘watchers’. Government documents reveal what it takes to follow suspects without being detected.

Do not use false beards, carry plenty of spare change for the tube, and if you have to tell a taxi driver to "follow that cab", be prepared to give him a big tip. That was the advice issued to rookie MI5 "watchers" whose job it was to covertly follow and report on suspected enemy agents.