CNN  — 

Authenticating paintings is no easy task. At times, it requires forensic specialists, academics, high courts and millions worth of legal fees. But for a portrait of Sir Francis Drake, proof of authenticity required a wart.

Several top experts reviewed the painting, which is set to go on auction in early July. Among other features, they examined Drake’s clothes, his hair and his eyes, according to Andrew McKenzie, head of Old Masters Paintings at Bonhams, the auction house selling the portrait of the privateer ennobled by Queen Elizabeth I.

“But it was the wart that clenched it,” McKenzie said.

The wart, which is located on the top of Drake’s nose, is absent in copies of other authentic portraits of Drake, said McKenzie.

A close-up of a painting of Sir Francis Drake by an unknown artist, whose authenticity was proven by a wart on his nose.

The painting, which is expected to fetch between £300,000 and £500,000, ($400,000 to $650,000) will lead the Old Masters sale at Bonhams in London on July 4. The portrait previously hung on the walls of Buckland Abbey, Drake’s former home.

It’s fitting that something as lowly as a wart verified the originality of the painting. Though Drake became a prominent member of British society, he began life as a humble farmer, a social ascent that conflicted with some of the elite of the era, according to Bonhams.

Referencing Drake, a 1952 letter to King Phillip II says : “The people of quality dislike him for having risen so high from such a lowly family; the rest say he is the main cause of wars.”

A portrait of (a wart-free) English explorer and adventurer Sir Francis Drake from around the time of his death in 1596.

The portrait – whose creator is unknown – dates from the mid-1570s, before Drake became the first Englishman to sail around the world and before he led the British navy’s defense against the Spanish Armada. It portrays Drake as a man of strength and high social standing, adorned in expensive armor.

“The portrait of Sir Francis Drake is a national treasure, capturing perfectly a brief moment in the history of one of our most renowned seamen,” said McKenzie.