Prince Charles has used money from people who die without wills or family in Cornwall to fund his own charities and to support bursaries at his old private school in Scotland.

As Duke of Cornwall, a title that already provides him with an £18m private annual income, a quirk of history means Charles becomes the owner of the assets of anyone living in the county who dies "intestate".

Last year that provided him with more than £450,000 and he is sitting on £3.3m in cash from many years of collecting Cornish legacies, latest accounts show.

In 2012, the benevolent fund he set up to use the money made one of its largest grants of £5,000 to the elite public school of Gordonstoun in Scotland where a place now costs £30,000 a year.

The biggest grant was £19,300 to Charles's charity Business in the Community, whose supporters include some of the biggest companies in Britain. Another £1,000 went to his London-based Prince's Foundation for Building Communities, which champions the prince's controversial ideas about architecture and planning.

The donations have drawn particular criticism in Cornwall, where there were calls for the inheritances to be channelled into the public purse as they are in the rest of England.

Burt Biscoe, a councillor in Truro, said Charles was "abusing the loyalty" of Cornish people and the "privilege" of receiving the intestate assets.

"If he is using this money to fund his own charities and his old school in Scotland then a further covert injustice is being prosecuted against Cornwall," he said. "Think what he could achieve if he gave that £450,000 to Cornwall every year … The area of giving should coincide with the area of taking."

John Angarrack, a Cornish nationalist who scrutinises the Duchy of Cornwall's activities, said: "We are one of the most impoverished regions in the UK and the money would be much better used here, where all sorts of youth projects are in need, than at Gordonstoun."

Rob Simmons, a member of the Cornwall search and rescue team who is standing for Mebyon Kernow, the Party for Cornwall, in Thursday's council elections, called on Charles to instead help the food banks in Camborne and Truro, which he said were regularly running out of supplies.

A spokeswoman for the Duchy of Cornwall said Charles's charities carried out work in the south-west and the money for Gordonstoun was intended to fund bursaries for Cornish children.

The fund's annual report states it is being treated as "a quasi-endowment" with income being distributed and the capital sum maintained in real terms. It said that grant giving was "constrained by the availability of sufficient suitable grant applications".

Charles has also been criticised for only distributing £100,000 – less than a quarter of the assets received from the deceased last year.

"Many people will be shocked to learn that Charles receives money from the dead, but we were always told that it went to charity," said Graham Smith, director of Republic, the campaign for an elected head of state. "Now we see that only a tiny proportion actually goes to good causes. Charles is sitting on those funds when they could be supporting the vital work of charities, many of whom are really struggling at the moment. The trust has only negligible costs and doesn't deliver any services so there's no reason why that money can't be used by voluntary and community organisations right now."

The Charity Commission's guidance on reserves states it will stage "regulatory intervention" if a charity's reserves are excessive. "While the funds remain in the trustees' hands, the charity's current users or beneficiaries – actual or potential – are not being as well served as they could be," it warns.

The benevolent fund made 151 grants last year. Others went to the Soil Association, the organic farming movement which Charles supports, the Dorchester arts festival near his Poundbury housing development in Dorset, the Friends of the Countryside, and several Cornish churches.