A British collector has returned a set of ancient bronze warrior helmets to Spain in a move the country has called the most significant restitution of stolen heritage since 2012.

The helmets were originally legally bought by the British collector Christian Levett, who – when he discovered they had been taken illegally from an archaeological site in Aranda de Moncayo, northern Spain during the 1980s – arranged to return them.

The items were made between 400BC and 200BC, and Levett worked with Spain’s ambassador to Unesco to return the helmets, signing an act of voluntary surrender after reading about the looting of the objects in the Spanish media.

Levett, a retired investment manager and founder of commodity hedge fund Clive Capital, originally paid £250,000 for the helmets – buying seven at auction in Munich in 2008 and one from a London dealer in 2009.

The items were on display at the Mougins Museum of Classical Art (MACM), an institution Levett opened in the south of France in 2011.

“In my personal opinion, when it’s clear beyond doubt, as in this case, that an object has been illegally looted and illegally exported from a particular source country, then I personally cannot see why anyone would want to retain that piece in their collection,” Levett told Artnet News.

The return is the most significant since 2012, when the US ordered the repatriation to Spain of a 15-tonne haul of coins (worth $500m) that was taken from the Nuestra Señora de las Mercedes wreck. The ship was sunk by a British ship near Portugal during a battle in 1804.

Spain’s ministry of culture has recently identified 681 shipwrecks that sank between 1492 and 1898 along the southern Atlantic coast of the US and in the Caribbean. Less than a quarter of them have been located and they are thought to contain food, clothes for slaves, weapons and religious objects.

Levett said the “initial cultural crime” had now been solved with the return of the helmets, but that he would now attempt to get his money back from the people who sold them to him.