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 guidance is contained in a booklet for recruits during World War Two, which will be on display at the National Archives in Kew, west London, in the spring.

To be successful in the Security Service, newcomers were told not to wear false beards, carry plenty of spare change for the Tube and to engage in a good cover story.

The instructions made it clear to not look towards spy films as inspiration as there was "very little glamour and much monotony" in the world of espionage.

The role of a watcher was to covertly follow and report on suspected enemy agents.

It was an assumption that successful watchers would be men of a "rather nondescript type" who would "look as unlike a policeman as possible", the booklet says.

Tactics included to double-up with other spies where possible and to keep 75 to 90ft away from the target while walking on the opposite side of the pavement on quiet streets when following a suspect.

Unlike in James Bond films, false hair was heavily frowned upon as it could be easily detected under harsh lighting such as in a restaurant, pub or train, according to the guidance.

"Be prepared for the suspect to board a moving vehicle such as a bus tram or train, or hailing a cruising taxi, or any other device to shake off a 'tail'," the booklet says.

Finally, if watchers do have to engage in a cab chase, the instruction is to be prepared with plenty of cash and a good cover story.

A suitable tale included a divorce case or an absconding partner, and to promise the driver a good tip.

Unlike government departments, MI5 is not subject to the Freedom of Information Act. It releases its archives as it chooses and some files are partially redacted.

Some of the newly released documents will feature in the forthcoming exhibition at the National Archives.

The Director General of MI5, Sir Ken McCallum, said: "While much of our work must remain secret, this exhibition reflects our ongoing commitment to being open wherever we can."