New rules could affect nine in 10 rental properties in duchies of Cornwall and Lancaster and at Sandringham

King Charles and Prince William could be forced to spend up to £10m upgrading hundreds of rental properties on their private estates to meet planned energy-efficiency rules.

The government has announced that landlords will need to ensure their properties have energy performance certificate (EPC) ratings of C or higher by 2030 – up from the current requirement of an E.

A number of idyllic-looking homes in estates owned by King Charles and Prince William do not meet energy standards for rentals

• Royal estates rent out homes that fail to meet legal energy efficiency standards

This summer, the duchy of Cornwall got a new-look website with a bold statement of intent at the top: “We’re on a mission to become a world-leading impact organisation.”

Updating the website will have been the easiest part of that mission. Upgrading hundreds of ageing, mostly rural, rented cottages, farmhouses and country houses will be one of the hardest.

Exclusive: More than 100 homes on King Charles and Prince William’s inherited estates have EPC ratings of F or G

• ‘It’s archaic’: the struggle to heat poorly insulated homes let by royal estates

More than 100 homes on the inherited royal estates of King Charles and Prince William fail to meet the legal minimum energy standards for landlords, the Guardian can reveal.

An analysis of a large sample of domestic lets on the duchies of Lancaster and Cornwall and the Sandringham estate found that one in five have Energy Performance Certificate (EPC) ratings of F or G.

MPs criticise 10-year deal struck in ‘blind panic’ to rent Dartmoor prison, where high levels of radon had been detected

A “catastrophic” decision by the Ministry of Justice to sign a 10-year lease on a prison where high levels of a poisonous gas had been detected is expected to cost the UK taxpayer more than £100m, parliament’s spending watchdog has concluded.

The public accounts committee said the 2022 deal to rent HMP Dartmoor from the Duchy of Cornwall was signed “in a blind panic” by senior civil servants looking to guarantee prison places.

Exclusive: Unknown culprit suspected of spraying glyphosate on protected trees hoped to stop peat erosion and flooding

Trees planted as part of a nature restoration project on Prince William’s land in Dartmoor national park have been deliberately poisoned with herbicide, sparking outrage and a hunt for the culprit.

The willow trees, on Duchy of Cornwall land, were planted as part of a project to stop peat erosion, store carbon and reduce the risk of flooding.