The government asked King Charles for permission to pass its post-Brexit “world-leading” Environment Act because laws requiring landowners to enhance conservation could affect his business interests. Environment minister Rebecca Pow wrote to the then Prince of Wales in 2019 to ask if he would accept section seven of the environment bill, which became law in November 2021. It refers to the need “to conserve the setting of land with a natural environment or natural resources or which is a place of archaeological, architectural, artistic, cultural or historic interest”. Those who breach the conservation covenant agreements could be subject to fines. When the act became law, ministers lauded it as a benefit of post-Brexit Britain. Then environment secretary George Eustace declared: “We are setting an example for the rest of the world to follow.” However, the government then missed its own legal deadline for setting what it termed as “ambitious” Environment Act targets. When they did emerge, months late, conservation charities said the targets for nature, clean air and water were underwhelming and would not deliver the promised environmental benefits by 2030. In the letters, revealed on Saturday and sent in October 2019, Pow informs Charles: “This bill contains measures on conservation covenants which affect the interests of the crown, the Duchy of Lancaster and the Duchy of Cornwall. Part 7 (conservation covenants) of the bill applies to crown land as it applies to any other land.” These conservation covenants are new agreements between the government and landowners in certain biodiverse and nationally significant areas requiring the landowner not to undertake certain environmentally destructive or polluting activities. The letters show that the prince’s private secretary, Clive Alderton, responded to Pow, giving his consent for the law, confirming that he was “content with the bill”. The Duchy of Cornwall brings in about £21m a year for the duke and his family. Revenues from the estate are passed to the Prince of Wales, now Charles’s son William, who uses them to fund his public, charitable and private activities. The duchy consists of approximately 53,000 hectares (130,000 acres) of land in 23 counties, mostly in south-west England, and in many areas of natural beauty and significance that are affected by environment targets. Charles has previously interfered in government affairs when it threatened to affect his businesses. Last year, the Guardian revealed that he exploited a controversial procedure to compel ministers in John Major’s government in 1992 to secretly change a proposed law to benefit his landed estate. Under the procedure, the late Queen and her eldest son were given copies of draft laws in advance so they could examine whether the legislation affected their public powers or private assets, such as his Duchy of Cornwall estate or the privately owned estate at Sandringham. This procedure is different from the better-known royal assent – a formality that makes a bill become law. The Guardian’s Queen’s consent investigations found that procedure had been used by the late monarch in recent decades to privately lobby for changes. During her reign, ministers were required to secure approval from her or her son for more than 1,000 parliamentary acts before they were implemented. They include leasehold reform legislation that became law in 1993, when there was evidence of Charles applying pressure on elected ministers to ensure an exemption to prevent his own tenants from having the right to buy their own homes. The Department for Environment, Food and Rural Affairs declined to comment. Buckingham Palace has been contacted for comment.