What is the name of this musical work?

I just heard the Greg Lake song ''Father Christmas'' on the radio. In it, there is a passage which I know has to have been borrowed from some classical music. The first time it shows up in the song, it's played on a lightly-amplified guitar. At the end, it sounds as if there's a small orchestra and choir involved in the proceedings. It sounds almost as if it could be a Tchaikovsky piece.