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The Government has announced an increase to the minimum Energy Performance Certificate by 2030

King Charles and Prince William could be facing a potential bill of up to £10million to bring hundreds of rental homes on their private estates up to the Government's upcoming energy-efficiency standards.

Laboue has announced rental properties now require an Energy Performance Certificate (EPC) of C or higher by 2030 – up from the current minimum of E.

An investigation by The Guardian has revealed nearly nine out of 10 domestic properties across the Duchy of Cornwall, the Duchy of Lancaster, and the Sandringham estate hold energy performance certificate ratings of D or below, meaning they will, by law, require upgrades.

Upgrade costs are estimated at as much as £10,000 per property, with fines of £30,000 for each non-compliant home still being let.

Energy performance certificates were introduced in 2007 as carbon emissions became an increasing public concern. They grade properties from A, the most efficient, down to G, the worst performers, offering an estimate of running costs and environmental impact.

A Government spokesman said: "Everyone deserves to live in a warm, comfortable home. Our plans for private rental homes to be EPC C by 2030 could lift up to half a million households out of fuel poverty by 2030, cutting bills to give families across the country the breathing space they need."

Across the country, slightly more than half of all privately rented homes currently fall below the C standard. The royal estates, however, perform notably worse than the national average.

The Sandringham estate in Norfolk fares worst of all. Of more than 150 EPCs examined by the newspaper, 99 per cent carried ratings between D and G, with not a single property achieving an A or B, while just one managed a C.

Three-quarters of the homes analysed sat at E – the bare minimum currently permitted for rental properties and two levels below the new requirements.

As for the Duchy of Lancaster, spanning roughly 18,000 hectares from north-west England to London, more than 90 per cent of its 200-plus assessed properties missed the C threshold, with nearly half rated E and more than a quarter at D. Upgrading those homes, it is said, could cost upwards of £2million.

Prince William's Duchy of Cornwall, covering some 55,000 hectares predominantly in the south-west, performed marginally better at around 80 per cent below C. Its improvement bill alone could reach £5million.

With the King's Foundation and William's Earthshot Prize, both the monarch and his eldest son have built reputations as champions of environmental causes, so the disappointing energy performance of properties on their private estates may be surprising.

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However, it is noted that the properties on the estates are predominantly older, frequently reliant on oil, coal, or wood for heating – fuels far less efficient than mains gas.

Only newer developments at Poundbury in Dorset and Nansledan in Cornwall, both part of the Duchy of Cornwall, consistently meet the proposed C standard.

The Duchy of Cornwall acknowledged the scale of the challenge, noting its "unique portfolio, including many historic buildings located in isolated rural and island locations".

A spokesman said: "We recognise that these challenges mean work does not always happen as fast as we would like, but we will continue to work closely with our tenants to identify effective and practical energy-efficiency improvements as quickly as possible."

The Duchy of Lancaster stressed it placed "great importance" on "being a responsible landlord", having spent more than £3million on residential repairs and upgrades in the past year.

Sandringham maintained its properties were "fully compliant with MEES regulations", while acknowledging exemptions applied to a small number of homes.

Jonathan Bean, a spokesman for the campaign group Fuel Poverty Action, said: "The royal duchies should be leading the way in upgrading all their properties to a minimum EPC of C by 2030 to protect people's health and the environment.

"Instead, they are dragging their feet and looking for loopholes. If even the richest landlords are allowed to get away with it, what hope is there of getting the rest to do the right thing?"

He urged ministers to "close all loopholes and increase enforcement efforts".