Mr Hutchinson, who is now 73, told The Telegraph he had received a telephone call out of the blue in January 1977 from the Queen Mother’s lady-in-waiting, while he was working at Taylor Hutchinson on Eton High Street. The company provided recording equipment to the music industry.

“It was a spur of the moment idea,” he said. “The Queen Mother wanted it to be a surprise, so it had to be done discreetly.”

Describing how he had hidden himself in the vestry with the “rather naughty” choirboys, he used a four-track tape machine with a wireless microphone so as not to arouse suspicion.

The recording was mixed at Pinewood studios in Buckinghamshire, before being pressed into an album with printed sleeve artwork. Some 120 copies were delivered to the Queen Mother to distribute, but were never made public and not sent to the Royal Archives.

By the year of the Golden Jubilee, the original recording had been damaged by water and it was not thought possible to salvage it.

But, Mr Hutchinson said, advances in technology had made it possible to improve the quality of the recording to such an extent it will now be released to celebrate the Queen’s Platinum Jubilee in 2022.

A CD of the album, as it was supplied to the Queen Mother, will sell for £35, and a limited edition box set for £350, through the website royalsilverjubilee.com.