A nature conservation trust will this week buy a famous Scottish island, in a move that would secure the future of one of Europe's most important bird colonies.

Ailsa Craig, a volcanic island immortalised in the memory of a million childhood holidays, climbs out of the Firth of Clyde 10 miles off the Ayrshire coast. For the last two years, its future has been uncertain since it was put up for sale for £2.5m by the Marquess of Ailsa.

The island is home to Europe's biggest gannet colony and an increasingly significant number of puffins. Its blue hone granite is considered ideal for the manufacture of curling stones, two-thirds of which are made from it.

It had been thought that the island would be snapped up. Any new buyer would also have been getting a ruined 16th-century castle, built to prevent the Spaniards grabbing the island, a lighthouse and four cottages for lighthouse men as part of the deal. More important, they would also be securing a rare opportunity to own one of Scotland's most recognisable jewels.

As the months passed, however, there seemed no prospect of a quick sale and the price dropped to £1.5m, amid concerns over the future maintenance of the island. However, Vladi Private Islands, the Hamburg-based estate agency acting for the marquess, said that it expects a British-based charitable environmental trust with a special interest in birds to submit a formal bid for the island on Monday.

Farhad Vladi, the agency's president, declined to disclose the trust's name, saying it would prefer to remain anonymous in the event that the bid is rejected. He also declined to say how much the trust plans to bid. "This is a serious and very positive bid, and I am very hopeful at this development," he added. "This trust is based in Britain and the patron is British, and they have other interests in Britain, but at this stage they do not want to reveal their identity."

The puzzle of the continued failure to find a buyer, which has perplexed scientists and environmentalists, can now also be explained. A small part of the island is already in private hands, deterring a number of potential buyers who might otherwise have been keen to make Ailsa Craig their hideaway paradise.

Just over a decade ago, Scottish Indian business tycoon Bobby Sandhu bought four cottages, a shed and a small area of adjacent land from the Northern Lighthouse Board for £85,000. He hoped to develop a five-star hotel, but claims that planning regulations made it impossible. He has now said he would be prepared to sell his interest on Ailsa Craig for £250,000, a price the estate agents were unwilling to meet.

Vladi said he had received expressions of interest from around 50 potential private buyers, of which around 15 he had deemed as serious. But he added: "As soon as people become aware that a plot of around one acre and some of the lighthouse keeper's buildings are owned by someone else, they are not interested."

Environmentalists will be delighted that the island is now expected to be taken over by a conservation trust. Ailsa Craig has had a dramatic and vivid history and was a haven for Roman Catholics escaping the rigours of the Reformation, who also hoped it could assist the Spanish in any future attempt to invade Scotland. Later it would become a prison. .

Previously, there has been concern at just how often little parcels of privately owned land in Scotland are being traded indiscriminately and with uncertain outcomes. Earlier this year the island of Tanera Mor in the Summer Isles, off the north-west coast of Scotland, was put up for sale at £2.5m after the local community rejected the opportunity of a buyout.

Land reform is again becoming a burning issue in Scotland. More than half of the country is owned by fewer than 500 individuals, much having been illegally annexed following the Scottish Reformation and passed down through the nation's richest families ever since.

Vladi Private Islands said the existence of another on-island landowner was not an issue for the conservation trust planning to bid. "This is not a problem for a not-for-profit conservation body. They are not buying the island for the same reasons that private buyers want an island."