For many people, an essential part of Christmas is the annual Festival of Nine Lessons and Carols in the chapel of King's College, Cambridge.

Alex Kirby: "The annual carol service marks the real start of Christmas"

It is held at 1500 GMT every Christmas Eve, and follows much the same pattern year after year, though it usually includes one or two new carols as well.

This year's service marks a double anniversary - the 80th service, and the 70th year it has been broadcast by the BBC.

The first service was held on Christmas Eve 1918, weeks after the end of the Great War's four years of mass slaughter.

It was introduced to the college by the then dean of King's, Eric Milner-White.

His time as an army chaplain on the Western Front had persuaded him that the church needed more imaginative forms of worship.

Audience of millions

And 10 years later came the first broadcast, a tradition that has remained unbroken except for one interruption in 1930.

A few years after that, the BBC began broadcasting the carol service on its overseas programmes, now known as the World Service.

Millions of people tune in across the world every Christmas Eve, by no means all of them expatriate Britons.

The broadcasts continued even during World War One, though conditions were spartan.

All the chapel's ancient glass had been removed from the windows, in case of bombing, and there was no heat in the vast building.

Rodney Williams, the soloist in 1954

For security reasons, the BBC could not even name King's as the place where the service was being held - though few listeners would have been in any doubt.

By tradition, the service begins with a lone choirboy singing the first verse of the carol "Once in royal David's city" unaccompanied.

Rodney Williams sang the solo in 1954, and still remembers the experience vividly.

The organist hummed him the first three notes, and then signalled him to start.

"When it was all over, I was able to relax and enjoy the rest of Christmas", he says.

Kept in suspense

Today, none of the choirboys knows until moments before the service begins which of them will be chosen to sing the solo.

The director of music at King's, Stephen Cleobury, says he has found that this is the best way to calm the boys' nerves.

Eric Milner-White always argued that the point of the service was not the carols, but the lessons.

"The main theme is the development of the loving purposes of God," he said, seen "through the windows and words of the Bible".

The carol service is meant for everyone

But for many people the music speaks more directly than any words, and the carols can appeal to listeners of all faiths, and of none.

The present dean of King's, the Reverend George Pattison, says the carol service was originally intended as a peace offering by the college to the city of Cambridge.

The words of the bidding prayer, said by the dean after the opening carol, include an invitation to remember "all those who rejoice with us, but on another shore, and in a greater light".

George Pattison hopes the carol service will speak in the same way today to a world still ravaged by war.