Bing Crosby found it "really difficult" to perform 'White Christmas' to the troops during World War II.

The late crooner - who passed away in 1977 at the age of 74 - achieved the highest-selling single of all time with the festive classic but his nephew Howard Crosby has revealed how his uncle once performed it in an open-air field in France in 1944 for thousands of soldiers and struggled to get through it.

He told Fox News Digital: "I asked Uncle Bing one time, 'What was the single most difficult thing you ever had to do in your career?' We were out playing golf one day, and I didn't know what he was going to say.

"I didn't know if he was going to say, 'Well, it was, you know, learning lines for the movies or working with a difficult director.

"He didn't have to think about it at all. He said, 'Well, 1944, we were over with the USO troupe.' And he said, 'We gave an open-air concert for 15,000 GIs and British Tommies in an open-air field in France.

"Dinah Shaw and the Andrews Sisters were there. and we had a lot of laughs and the boys were having a wonderful time, great fun.' But he said at the end of the show, 'I had to sing "White Christmas." And I had to get through the song with 15,000 guys in tears and not break up myself.

"And a lot of those boys died the next week in the Battle of the Bulge."

Almost a decade later, Bing starred in 'White Christmas' as a solider who performed the song to the troops in 1944, but the song was initially written for 'Holiday Inn' in 1942.

At the time, composer Irving Berlin was said to be "very nervous" about the now-festive standard, but Bing himself had no worries about it at all.

Howard said: "But when they got to 'White Christmas' and Irving Berlin played that, supposedly Bing said to him - because Irving was always very nervous about whether the songs would be successful or people would like them - apparently Bing said to him, 'Don't worry about that one, Irving, that song is going to do just fine."