St Helena, a tiny volcanic island in the South Atlantic Ocean, is seeking a new director for its national trust. It must be among the best jobs going: a posting to one of the world's most spectacular locations, rich in historical buildings, wildlife and in sun, sea and fresh air, with a negligible crime rate, fine food, friendly people and no traffic.

But there is trouble in paradise. The island's splendid isolation – it is one of the most remote inhabited places in the world – looks set to change for ever with the construction of an airport and a 300-acre luxury hotel, villa and golf course complex. The scheme is dividing islanders and causing alarm among wildlife and heritage conservationists.

British taxpayers are set to spend up to £300m on a new airport, to be sited amid one of the island's most sensitive ecological zones. The resort complex would straddle a large portion of a key environmental area.

A single bid to build the airport and runway – a cornerstone of the British government's strategy for ending its financial support of one of the last bastions of empire – has been submitted to the Department for International Development (DfID) by a South African construction firm, Basil Read.

But the prospect of the arrival of the jet age on the island horrifies some heritage experts. Martin Drury, former head of Britain's National Trust and a highly respected voice in international conservation, told the Observer that the airport and hotel scheme were likely to destroy much of St Helena's time-capsule character.

Only 10 miles by six, St Helena is 1,200 miles from Africa, 1,800 miles from South America, two days by boat from the nearest inhabited island, Ascension – a mere 700 miles away – and some 14 days by boat from the UK. In recent years its population has fallen from just under 6,000 to 4,900. In its heyday, more than 1,000 ships a year called at St Helena. The island's only regular visitor these days is the ageing Royal Mail Ship, RMS St Helena, which docks once a month.

As modernisation looms, dissent is in the air. This April, St Helenians, known as "Saints", protested against taxation reforms agreed between DfID and the St Helena government, aimed at paving the way for large-scale tourism. Some demonstrators called for the sacking of St Helena's governor, Andrew Gurr.

The St Helena airport scheme was given the green light last July by the international development secretary, Andrew Mitchell. The bill for the airport – between £100m and £300m – will be footed by Britain. The leisure complex, to be built only if the airport scheme goes ahead, would be funded by a luxury holiday development consortium, the St Helena Leisure Corporation (Shelco).

The prospect of large-scale construction work, new roads, buildings and passenger jets does not bode well for St Helena's flora and fauna, whose diversity astonished even Charles Darwin. Despite the island's wealth of natural and historical treasures, it has no legally protected areas. And the population of the wirebird, the island's national bird, is down to its last 300.

"The [proposed] resort would be on one of the green bits of the island," Jamie Roberts, outgoing director of St Helena's national trust, told the Observer. "This also happens to be one of the best sites for wirebirds … It's a big area for St Helena, and especially important as it is productive agricultural land."

Nevertheless, DfID has trumpeted mass tourism. "The UK government believes a new airport is the best way to bring new financial opportunities to the island, not least a boom in tourism," it stated. "With an airport it is estimated that more than 29,000 tourists will visit each year." Industry insiders predict that will reach 50,000 visitors a year.

Tourist cash would be welcome. Wages on St Helena average less than £4,000 a year. Britain's annual support is expected to exceed £30m this year.

Despite environmental concerns, the drive to build on the island appears unstoppable – and has high-profile supporters. Last month a co-founder of Shelco, civil engineer Sir Nigel Thompson, visited St Helena. His green credentials include a spell as chairman of the Campaign to Protect Rural England.

Much of the momentum behind the airport plan derives from the long-standing interest in the island shown by billionaire businessman Lord Ashcroft, the Conservative donor and the party's former deputy chairman and treasurer. The Observer contacted Ashcroft's office last week, but received no reply.

Roberts said the problems facing St Helena were serious. "We've had a dialogue with the UK government in recent years to get them to see that they have a responsibility for looking after this British heritage, but we've only had limited progress," he said.

"The site of the proposed airport is incredibly important for native invertebrate animals … St Helena has about 400 species that occur nowhere else in the world. A lot of them live on that plain."

Martin Drury, who helped to set up the St Helena National Trust in 2002 and is now chairman of the Landmark Trust in Britain, said the model should be Lundy Island in the Bristol Channel – "adapted for tourism without new development". "People can play golf anywhere," he said. "The whole point of going to St Helena is to get away from all that."

DfID said: "An airport is the only way to save the island from permanent decline. The RSPB is helping the St Helena government to create a special habitat for local birds. National protected areas are also being established for wildlife."

"The development plans approach the matter from the wrong angle," said Drury. "They started with the idea for a big airport … that's led to all sorts of consequences which will, I think, eventually destroy the island."

Many islanders resent the pace of economic reforms. "Some families go back to the late 1600s on the island," said leading Saint Basil George, who believes the narrow time frame of the fiscal reforms has led to a rise in the cost of living. "If people think they don't have a stake in the place, that can upset the norms that hold our society together. That's something of concern," he said.

"What's very special about St Helena is that we came from a very divided society, different racial groups, but that we've come to be integrated, with a common identity. We've been through some tough times."