LONDON—Wartime leader, Nobel Prize-winning historian, amateur painter—and perhaps a poet, too.

A rare piece of verse by Winston Churchill is set to be auctioned, offering a glimpse into his patriotic and militaristic frame of mind. The 40-line work is written by hand, signed but not dated; experts believe it was written around 1900.

Churchill wrote poetry while a student but experts say this poem—called "Our Modern Watchwords"—is unusual because it was written when Churchill was an adult.

One section describes the mood before a naval battle: "The silence of a mighty fleet portends the tumult yet to be."

It will be sold by Bonhams auction house in April and is expected to fetch between 12,000 and 15,000 pounds ($18,800 and $23,500).