Britain’s famous “speaking clock” ticked round to a major milestone on Sunday – its 75th birthday.

People who need to know the time will be able to dial the service’s number – 123 – to hear a special birthday message.

Now widely considered a national institution and part of Britain’s heritage, the original speaking clock was introduced in 1936 for people who did not have a watch or clock to hand.

Initially the accuracy of the clock was one-tenth of a second, but today it is accurate to within five thousandths of a second.

Even Big Ben uses the service to check its accuracy. Around 30 million calls every year are made to the speaking clock, which is officially called Timeline.

The “voice” of the clock is almost as famous as the service itself, with only four permanent voices ever used. However, celebrity voices have been heard on occasion to help raise money for charity.

In 2009 Gary Barlow, Cheryl Cole, Chris Moyles, Kimberley Walsh and Fearne Cotton leant their voices to the Speaking Clock in aid of Comic Relief.

David Hay, head of BT Heritage, said: “Seventy-five years ago BT’s technology created the speaking clock, which remains a much loved part of British life today. Celebrating its diamond jubilee demonstrates BT’s determination to preserve the heritage of the world’s oldest communications company.”

The clock was designed and built at the Post Office Engineering Research Station in north London.

Time announcements were automatically co-ordinated on the hour with Greenwich meantime signals.

In order to reach the service, subscribers would dial the first three letters of the word ‘time’, because telephone dials at the time included letters as well as numbers to aid automatic calls.

Dialling T. I. M. helped give the clock its common name, ‘TIM’.