At the third stroke the speaking clock turned 75, precisely, and is still going strong.

The iconic telephone timepiece celebrated its birthday yesterday and is still used by more than 30 million people a year for an accurate time check.

It has changed the fortunes of a pair of Brighton actors who have become two of the best known voices in Britain.

Voice-over artist Sara Mendes de Costa, from Hove, has been the voice of the speaking clock since 2007.

She took over from actor Brian Cobby, 81, who lives a few streets away, and whose silky tones had given the time to millions of callers since 1985.

Mr Cobby was famously the voice for the “5-4-3-2-1 Thunderbirds are go”

countdown in the Thunderbirds television series and was the first male voice of the speaking clock.

Ms da Costa, 44, is not only the voice of time for the BT clock but is also the voice of the Tardis computer in the Dr Who game for the Wii console.

She said: “It was sort of the Voice of Time meets the Time Lord, which was rather nice.

“Being the voice of the speaking clock has given me a boost in the right direction to do other things.

“Before I won the competition in 2006 I was running my own telemarketing company and hated it.

“I decided I wanted to become a writer and started to do voice-overs to earn some money.

“I was building that up when the competition for the speaking clock came up and it has taken off from there.”

BT launched the search for a new voice for the clock through Children in Need in 2006.

Ms da Costa beat more than 18,400 other hopefuls who recorded a time check for the competition.

Her voice stood out to the judges who included her predecessor, Brian Cobby, who had no idea she lived so close.

He said: “Sara's voice stood out a mile. We all voted for her.”

There have only been four official voices of the speaking clock since it started in 1936.

The speaking clock is accurate to within five thousands of a second It is used by the operators of Big Ben to check the accuracy of the iconic clock at Westminster.