London, UK - Britain's royal family is facing calls for more transparency and reform of their private estates after an investigation alleged they have been profiting from public bodies while benefiting from major tax exemptions.

The UK media probe also accused the estates of King Charles III and his eldest son Prince William of making big profits from charities and individual renters while in some cases failing to meet environmental standards.

The centuries-old estates – the Duchies of Lancaster and Cornwall – have made millions of pounds (dollars) from lucrative deals with the publicly-funded National Health Service (NHS) and other cash-strapped ministries, according to the investigation.

Both estates – portfolios of land, property, and assets across England and Wales held in trust for the king and his heir – are exempt from paying UK corporation or capital gains taxes.

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The extent of their holdings and commercial deals, such as lease agreements, is not publicly disclosed.

But the probe by UK television network Channel 4's Dispatches program and The Sunday Times claims to have uncovered them for the first time.

It has prompted calls for a review by parliament as well as demands by pro-republicans for the duchies to be abolished.

Norman Baker, a former lawmaker from the centrist Liberal Democrats party and longtime royal critic, told AFP the findings confirmed his view that the royals were "taking the public for a ride."

"These are Crown lands which belong to the public... all that money should be going into the Crown Estate, which is a public asset," he said.

The royals have long maintained that profits from the duchies fund their public, charitable, and private activities.

The duchies, owned by the monarchy since the Middle Ages, were not part of a 1760 agreement which sees the monarch's Crown Estate profits surrendered to the government.

15% of those profits are returned as a Sovereign Grant, which pays for official engagements, staff salaries and the royal palaces' upkeep.

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Next year, the grant will total £132 million ($171 million).

Baker notes the vast private estates were not included in that arrangement because, at the time, they did not generate much income. But two centuries on, their assets are worth £1.8 billion ($2.3 million), with profits topping £50 million ($65 million) in 2023, according to their annual reports.

It has helped keep Charles on The Sunday Times Rich List, which ranks the 1,000 wealthiest people or families in the UK, with an estimated worth of £610 million ($787 million).

Lucrative deals leasing land to the crisis-hit NHS, armed forces, publicly-funded schools, charities, and renters have boosted his bottom line, according to the media probe.

In one example, Charles' estate will earn nearly £12 million ($15 million) over 15 years storing a new fleet of electric ambulances owned by a London hospital in one of its warehouses.

In another, William's estate will net £37.5 million ($48 million) over 25 years from the Ministry of Justice for leasing the currently empty Dartmoor Prison.

Graham Smith, head of anti-monarchy pressure group Republic, said the investigation "shows how the duchies are doggedly pursuing profit at every turn, at huge expense to the public and charities."

The duchies have denied any wrongdoing.

It is not the first time that the royals have stoked controversy.

In 2006, an influential parliamentary committee pressured the government over why they benefit from major tax exemptions.

Baker said the Public Accounts Committee should refocus on it.

"It's only under pressure like that that they'll change," he added. "Unless they're forced into doing something, they won't do it."

The former MP believes the royals are "in danger of losing public support big time" over the issue.