The organisation which oversees many of Scotland’s historic sites needs to "drive forward an ambitious programme of change", a review has found.

Holyrood’s Presiding Officer George Reid identified the Trust has been heavily over-governed for far too long and its management has been held back due to a top-heavy structure. As a solution he has identified the body should slash trustee numbers from 87 to 15 and elect a new board.

The National Trust for Scotland plays a crucial role in protecting and promoting cultural heritage and oversees more than 100 sites, including Crathes Castle in Aberdeenshire and Culloden Battlefield.

His five recommendations to make the National Trust for Scotland "fit for purpose" also included putting a new board in place by next April, a full audit of the organisation’s properties and a new "mission statement".

Mr Reid who led the strategic review told STV News: “The board conceded the trust was not sustainable and they’d known that for five years – things were uncomfortable.

“It was living a hand-to-mouth existence. It was living by selling off assets, it was living by delaying projects and it was living by lucky win-fall assets and that’s not a sustainable way for the future.

The key thing is they must know how much maintaining their present estate costs. That will sort the property situation, which has been the big pressure on finances. And at the end of the process of reform, which will take four to five years, there will be new legislation

“I think people realise the time for change has come and I’m pretty sure reform will happen, the trust will be secure and fit for purpose."

National Trust for Scotland chief executive Kate Mavor, said: “George Reid and his review team have correctly identified the trust has been heavily over-governed for far too long and that its management has been held back through the existence of such a top-heavy structure.

“For our part, we firmly believe the reforms to the governance of the charity can be introduced within the timescale set. We also believe change is essential to take the National Trust for Scotland forward and meet the important conservation challenges ahead.”

The chief executive added that the trust's latest accounts showed it to be in a "more stable financial position".

National Trust for Scotland president, The Duke of Buccleuch, said: "We face urgent challenges, but thanks to the speed with which Mr Reid worked, there is now a clear route map for reform to put to National Trust for Scotland members at their AGM in September."

Welcoming the report, Culture Minister Fiona Hyslop, said: "The immediate priorities for the trust are to manage its functions more effectively to protect the national heritage that has been entrusted to it.

"The Scottish government believes there needs to be more effective and sustainable partnerships developed by the trust with other heritage organisations such as Historic Scotland and I am keen to see that fostered."

The historic environment sector is said to be worth more than £2.3bn to the economy and supports 40,000 jobs.