It is a piece of Scotland’s bloody clan history that has remained hidden for more than 400 years.

But a rare find during an excavation at Dunyvaig Castle on Islay has uncovered the seal of Sir John Campbell of Cawdor.

The artefact dates back to 1600s when the Campbell and MacDonald clans were locked in a violent and bitter feud over Scotland’s islands.

Buried below mounds of rubble, the find - described as “remarkable” and “extremely rare” by archaeologists and historians - was discovered on a hidden clay floor at the historic site.

Archaeologist Dr Darko Mari?evi?, director of the excavation at Dunyvaig, said: “This is a remarkable find. Not only is it a beautiful and well-preserved object, but it comes from the floor of a building that we can now confidently date to the Campbell occupation.

“So buried below this floor, we will have the story of the MacDonald’s – the Lords of the Isles – to reveal.”

Roddy Regan, an archaeologist at Kilmartin Museum, added: “Seals are extremely rare finds. This discovery conjures up an image of a Campbell garrison fleeing from the castle when under attack, dropping and losing one of their most precious items, or maybe the seal had once been hidden within a wall niche and long forgotten.”

Once used to sign and seal charters and legal documents, the seal is a circular disc of lead which carries the inscription IOANNIS CAMPBELL DE CALDER (Calder was the original spelling of Cawdor).

It carries the Cawdor coat of arms with a galley ship and a stag and is dated 1593.

The Campbells and the MacDonald’s fought over Dunyvaig in the early 17th century, with a series of sieges and bombardments of the castle until the Campbells finally prevailed.

Sir John took ownership of Islay in 1615.

Mr Regan added that the seal could have been lost in a later raid in 1646 when Alasdair MacColla, a descendent of the MacDonalds, reclaimed the castle.

He said: “Alasdair MacColla, a descendent of the MacDonalds, retook the castle and installed his elderly father, Colla Ciotach to defend it.

“The castle was immediately besieged again.

“We may have Colla Ciotach’s hasty defences in the form of turf walls built above the already ruined stone walls of Dunyvaig, before he was forced to surrender in 1647 and then hanged from the castle walls.”

The excavation at Dunyvaig is being undertaken by charity Islay Heritage in partnership with the University of Reading.

A team of around 40 experts, including leading archaeologists, geophysicists, scientists and environmentalists, are almost at the end of an initial three-week dig at the castle, which sits in Lagavulin Bay, with further projects planned for next year.

Organisers were keen to use the excavation as a teaching project for current archaeology students and it was one such student, Zoë Wiacek, from the University of Reading, who discovered the seal.

She said: “I removed a piece of rubble and it was just sitting there on the ground. I immediately knew it was an important find, but had no idea what it was.

“I called over my trench supervisor, and when it was lifted, the soil fell away to show the inscription. Then everyone became excited.

“I am so proud to have found something so important for the project and for Islay.”

Professor Steven Mithen, chairperson of Islay Heritage and director of the Dunyvaig Project added: “Coming towards the end of the dig, after the team had worked so hard to move huge amounts of turf and rubble, this has been a thrilling discovery.

“We have found a piece of Islay’s past and Scottish history. We can’t wait to start digging again in 2019.”

Islay Heritage hope to raise further funds to allow excavation work to continue at Dunyvaig over the next five years.