Royal courtiers privately put pressure on the Welsh government to ensure that King Charles could not be prosecuted for rural crimes under a new law that ministers had drawn up, documents reveal.

The elected minister in the Welsh government who is its chief legal adviser was “not happy” that the king was to be given the special exemption from prosecution but agreed to it last year.

A Buckingham Palace official phoned the Welsh government to secure the assurance under an archaic custom that requires UK parliaments to obtain the consent of the monarch to draft bills before they can be implemented.

Under the mechanism, ministers notify the royal family of specific clauses in draft laws that may affect their personal wealth, their private property or their public functions. The ministers ask the monarch to approve the laws before they can be passed.

Investigations by the Guardian have shown that the late queen used her privileged access to draft legislation to secure changes that protected her private interests or reflected her opinions. In one recent example, her lawyers lobbied Scottish ministers in 2021 to change a draft law to exempt her private land from a major initiative to cut carbon emissions.

The use of the consent mechanism has been criticised as “undemocratic”. It has been in force in Westminster since the 1700s and has been extended to the Scottish and Welsh parliaments.

During the queen’s 70-year reign, she vetted more than 1,000 draft laws before they were approved by elected politicians. Those included bills that affected the her personal property such as her privately owned estates in Balmoral and Sandringham.

The mechanism has continued seamlessly into the reign of Charles. Ministers in Westminster, Scotland and Wales have been required to obtain his consent to 20 laws since he came to the throne in September 2022.

Buckingham Palace refused to say whether the king had asked for any changes to these laws before approving them. One was a bill that was formulated by the Welsh government to reform agricultural practices.

On 1 June last year, the Welsh government noted in an internal memo that its lawyers “had been contacted by Buckingham Palace officials who have sought an assurance that Welsh ministers will take into account conventions regarding prosecuting the crown when making regulations under this bill”.

In an email the following day, Welsh officials noted that Mick Antoniw, the Welsh government’s counsel general – the equivalent of the attorney general in Westminster – was “not happy with the exclusion”. However, he “recognises the ongoing convention and therefore” agreed to it.

This was a reference to an ill-defined convention under which criminal and civil proceedings cannot be brought against the monarch as head of state. The monarch has been given personal immunity from swathes of British law, ranging from animal welfare to workers’ rights.

However, an investigation by the Guardian has previously highlighted the extent to which this practice gives the monarch immunity for his conduct as a private citizen, affording protection to the king’s privately owned assets and estates.

More than 30 laws stipulate, for example, that police are barred from entering the privately owned Balmoral and Sandringham estates without the king’s permission to investigate possible crimes, including wildlife offences and environmental pollution. No other private landowner in the country is given such legal immunity.

In the case of last year’s Agriculture (Wales) Act, the monarch was exempted from regulations relating to the marketing of agricultural products, the disposal of carcasses and the disclosure of information to the Welsh state. Police are also unable to gain automatic entry to the king’s private property portfolio under that part of the act.

According to Buckingham Palace, the royal household rang the Welsh government to ensure that “as a matter of legal correctness” the monarch could not be prosecuted under the act.

A palace spokesperson said the convention had to be maintained as the draft act contained a particular type of legislation that would not rule out the possibility of a prosecution.

The spokesperson added: “At no point were any objections raised by the Welsh government, either formally or informally.”

A Welsh government spokesperson said: “The immunity of the monarch from prosecution is a long-established principle.” They declined to comment further.

Charles approved the bill on 20 June 2023, according to the documents released under the Freedom of Information Act. The documents do not specify which of his properties would be affected by this act.

Other laws which have been screened by Charles under the consent mechanism include legislation relating to the rents that UK landowners can charge mobile phone companies for putting up masts on their land and the management of Scottish private trusts. Trusts are widely used by the Windsors and can help the rich to shield their assets from public scrutiny or tax.

Charles gave his permission to a Scottish act that froze the rents for tenants in private properties and a Westminster act that required landlords to produce an electrical certificate in their rented homes. Charles rents out more than 300 homes across his Balmoral and Sandringham estates.

The palace spokesperson said: “King’s consent is a parliamentary process and His Majesty has granted consent on each occasion it has been requested by government.”