The National Trust for Scotland has announced a £2.5 million upgrade of flagship property Culzean Castle ahead of the charity's annual general meeting amid union concerns over wider revamp plans.

The trust said the investment is the first phase of a plan to boost visitor numbers to the historic Ayrshire estate.

It comes as the union Prospect criticised potential jobs losses and said 82 per cent of staff "have no confidence" in NTS restructure plans which the trust said will mean £17m investment in conservation in Scotland.

The NTS said it will "transform the welcome" at Culzean Castle, a first-wave priority property along with Newhailes and Brodie Castle, with a redesigned arrival area, bolster activity options with an expanded trail network and reinstate areas of the historic designed landscape protecting the environment of the Grade A listed buildings and Robert Adam’s original picturesque design.

Simon Skinner, NTS chief executive, said Culzean is the "first tangible example" of a new approach that includes investment to attract more visitors and make the most of the assets.

However Prospect negotiator Ian Perth said there are still 140 jobs at risk and warned the work of the trust could be unsustainable in the long term.

He said: "While we have made significant progress in negotiation with the rust, our members have been frustrated at the lack of clarity available.

"While we fully accept an organisation like NTS must be financially prudent, we can’t accept that making people redundant and replacing them with outside contractors is the way forward for the charity.

He said "questions remain unanswered", adding: "Our members are seeking answers to basic questions about how the charity will operate into the future and what work the trust will no longer carry out, and the answers have not been clear.’’

Mr Skinner said: "It is conservation which is at the core of this change programme.

"Failing to address the trust’s long-standing issues is not an option.

"We need to change and we need to change now.

“Early information from an independent survey shows that in fact the majority of staff do support the need for strategic change and are clear where they can find information about the plans."

He went on: "Outsourcing of services has been raised several times – but most of our professional conservation services are already commissioned from outside providers.

"The new model will be no different in that respect."