Thirty years ago, a painting by the British artist Sir William Beechey was sold as “portrait of a man”.

The anonymous buyer, however, knew precisely who the unnamed man in the picture was: Pascal Paoli, the 18th-century Corsican independence leader and icon of the Enlightenment.

Since that sale in April 1994, the Beechey portrait of the man credited with giving his Mediterranean island a modern written constitution – one that would later inspire American revolutionaries – has been held by a private collector on the Mediterranean island, unseen by the public.

It is now up for auction once again, this time correctly titled, in Corsica on the 300th anniversary of Paoli’s birth.

“There are a few portraits of Paoli in museums but this one is in my view the most important,” the auctioneer Vincent Bronzini de Caraffa told the Guardian.

He said the painting was “far more than a work of art” and “touches on the identity of our island and the ideal of European freedom”, showing a figure who in his lifetime was admired by the philosopher Jean-Jacques Rousseau, the writer Samuel Johnson and was a hero to a young Napoleon Bonaparte.

“I’m from an old Corsican family and I feel invested in a duty to respect the cultural and memorial significance of this work of art,” De Caraffa added. “It is being sold in Corsica so it can be seen by Corsicans before it perhaps leaves the island.”

Paoli, who Corsicans nicknamed U Babbu di a Patria (Father of the fatherland), has been largely forgotten outside his homeland, which at the time of his birth was under Genoese rule. As the island’s elected leader between 1755 and 1769, he declared Corsica an independent republic, founded a university, introduced a system of representative democracy and wrote a constitution that drew heavily on Enlightenment ideas.

George III by Sir William Beechey. The king sought Pascal Paoli’s support against the French. Photograph: Alamy

When French troops invaded in 1768, Paoli oversaw the resistance movement whose defeat a year later led to him being forced into exile in England for the first time. In London, Paoli met King George III and was given a royal pension after agreeing that if he ever returned to power on Corsica he would defend English interests against the French.

Paoli did return to preside over the short-lived Anglo-Corsican kingdom between 1794 and 1796 during which the island came under the protection of the British monarch during the wars that followed the French Revolution. Corsica was finally reoccupied by France and became one of its provinces.

Paoli’s British sympathies and opposition to the guillotining of Louis XVI led to him being declared a traitor and again forced into exile. He died in 1807 and was buried in Old St Pancras churchyard in London before his remains were transferred to Corsica in 1889. He has a bust in Westminster Abbey.

The portrait, measuring 1-metre 26cm high was completed in the years before Paoli’s death.

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“It is the silent witness, the last message of an Enlightenment man who has become, through this portrait, one of the most noble faces of European liberty,” De Caraffa said.

Eric Turquin, an art expert and French auctioneer, said: “Fifteen years ago this painting would have gone directly to London to be sold. London was the centre of the old masters’ market. Brexit was the last straw; for those in my field it was a major disaster.”

“The red tape has multiplied fivefold. London is still a major old masters’ centre but it is really suffering.”

De Caraffa said had things turned out differently: “Corsica could have been English.

“Corsica has always had good relations with England and many people have a great deal of affection for the country.”

The painting will be displayed at the Hôtel Ostella in Bastia on Thursday and Friday before the auction on 13 December.