In which year was the phrase "Mind the gap" first used on British train stations?

"Mind the gap" is an audible or visual warning phrase issued to rail passengers to take caution while crossing the horizontal, and in some cases vertical, spatial gap between the train door and the station platform.

The phrase was first introduced in 1969 on the London Underground in the United Kingdom. It is today popularly associated with the UK among tourists because of the particularly British word choice (this meaning of the verb mind has largely fallen into disuse in the US).

The phrase "Mind the gap" was coined in around 1968 for a planned automated announcement, after it had become impractical for drivers and station attendants to warn passengers. London Underground chose digital recording using solid state equipment with no moving parts. As data storage capacity was expensive, the phrase had to be short. A concise warning was also easier to paint onto the platform.