An airport built on a remote South Atlantic island by the British government – at the cost £285m of public money – cannot be used because planes landing there are blown off course, Whitehall’s spending watchdog has found.

The development on St Helena was approved in 2010 by the then development secretary Andrew Mitchell and was one of the biggest single government investments ever made in a UK overseas territory.

The cliff-top landing strip was due to be opened last month as part of a plan to end the island’s dependency on UK government subsidies.

A report by the National Audit Office has found that safety concerns relating to wind-shear and turbulence have led to the mothballing of the airport. Wind-shear is a sudden powerful change in wind direction that can destabilise large aircraft and has been responsible for crashes around the world.

The NAO said the Department for International Development (DfID) had subsidised St Helena with £28m in 2015-16 and was expected to hand it a total of £667m by 2043, when subsidies are due to end.

However, the airport delay could add to those costs, the report released on Thursday concludes. Auditors are concerned that without an airport the island will not be able to attract enough tourists to make it self-sufficient.

Responding to the report, Diane Abbott, the shadow international development secretary, said: “It is extremely concerning that such a huge investment has been mismanaged by this government … ministers must urgently clarify the additional impact of the delay on St Helena.”

Tim Farron, leader of the Lib Dems, questioned why the government had failed to commission studies on windshear before the money was spent. “This is a serious misuse of the aid budget which is meant to be helping the poorest in the world, and will present an open goal to those who looking for cynical opportunities to criticise international aid,” he said.

Amyas Morse, head of the National Audit Office, said the financial stability of the island would depend on a number of factors including tourist numbers and how much they spend. “The airport’s planned opening date in May 2016 has been postponed as outstanding safety concerns are addressed, potentially adding to the project’s cost and delaying its benefits,” he said.

The island, which is home to around 4,000 people and is located 1,200 miles west of Angola, is known for being where Napoleon was exiled in 1815 after his defeat at Waterloo. He died there in 1821. It can only be reached by sea and has relied upon the ageing Royal Mail ship St Helena which is soon to be retired.

The NAO said the overall success of the scheme depended on attracting sufficient tourists to the island as well ensuring that they spent enough once they were there. While the island had enough hotel places for the numbers projected to visit during the first six years, the island government had admitted that it needed to improve the accommodation’s quality.

DfID and St Helena’s government are looking at the options for dealing with the problem caused by the wind conditions but have yet to agree on a solution.

The former Tory party treasurer Lord Ashcroft, who was a vocal supporter of the airport in 2010, wrote on the ConservativeHome website last week that he was forced to abandon a planned visit to the island because of “serious concerns that the airport is too dangerous to use”. The Tory-led coalition approved the scheme with £250m from the development fund – the largest single investment it has made in overseas territories.

A DfID spokesperson said that some planes had been able to land on the island, including an airplane which successfully evacuated a sick child to Cape Town on Saturday. “As the report points out, the UK supports its overseas territories in line with international law,” she said. “We are helping St Helena overcome the challenge of being one of the most remote island communities in the world so it can develop its tourism industry and become financially self-sufficient, making it less dependent on aid.”