The Treshnish Isles are thought to be where the monks of Iona buried their library for safety in the wake of the Reformation of 1560.

The treasure has never been recovered and now the islands, which stretch for six miles off the west of the Isle of Mull, are to be sold.

They could change hands for more than �400,000.

It is believed the monks' trove could have been hidden on one of the five main islands, Cairn na Burgh More.

The Treshnish Isles lie to the west of Mull

If discovered, the collection of beautifully illustrated monastic books would be worth millions of pounds.

The islands, which are uninhabited, are also renowned for their bird life and have been designated as a Special Protection Area for Birds by Scottish Natural Heritage under the European laws.

The chain has five main islands - Lunga, Cairn na Burgh More, Cairn na Burgh Beg, Dutchman's Cap and Fladda - and covers about 320 acres.

An internationally important population of storm petrels live on the isles.

Barnacle geese, another internationally important bird group, make the islands their home in the winter.

Black guillemot, puffin, kittiwakes and fulmar plus one of the world's rarest birds, the Corncrake, also inhabit the chain.

The isles are a bird haven

Lunga is an important breeding ground for grey seals, but only three land mammals have been recorded - mice, rabbits and domestic sheep.

The mice may be descendants of rodents which lived in a village during the middle of the last century according to Dr Frank Fraser Darling, whose notes were eaten by mice during his stay on Lunga in 1937.

All the Treshnish group are of volcanic origin and are marked as a Site of Special Scientific Interest.

The islands are rich in history. When they were acquired by the Lord of the Isles in 1354 they were on the frontier between the Nodoreys and Sudoreys - the northern and southern isles into which the Hebrides were divided under the stormy rule of the Vikings.

A conservation body may bid for the isles

On Cairn na Burgh More there are remains of a Castle, originally built by Norsemen. This later became a Maclean clan stronghold where unwanted wives were kept.

During a clan feud, Maclean of Lochbuie is said to have been imprisoned in the castle with the ugliest woman on the island.

But that did not stop him giving her a son who eventually won back his heritage.

The chain is being offered for sale - on behalf of a private family trust, which has owned them for the last 60 years - Knight Frank estate agents.

Spokesman Colin Strang Steel said he did not think the chain would be bought by someone who planned to live there.

"They are quite unusual and will be purchased either by a conservation body or somebody with an interest in protecting their heritage," he said.