Judgment due on whether government illegally blocked publication of letters to ministers in which prince sought to change policies

The supreme court’s judgment on whether the government unlawfully blocked the publication of a series of secret letters written by Prince Charles is due to be made public on Thursday next week, court officials have announced.

For 10 years, the government, with the support of Charles, has been resisting a freedom of information request by the Guardian to see the letters sent by the prince to ministers in which he sought to change policies.

Thursday’s ruling appears to be the final stage of the dispute. A defeat for the government would clear the way for the publication of the correspondence, which, according to ministers, contains the prince’s “most deeply held personal views and beliefs”.

The supreme court’s website lists the case as due for ruling on Thursday 26 March in courtroom one at 9.45am. The proposed bench for the ruling is Lord Neuberger – the court’s president – Lady Hale and Lord Mance.

The lengthy legal battle reached the highest court in the land in November when seven judges in the supreme court started examining whether the government had the legal power to veto the publication of the letters.

Dominic Grieve, the then attorney general, had overridden a court and issued the veto in 2012 because he believed that disclosure of the letters would “seriously damage” Charles’s future role as king if they were made public.

The heir to the throne has for many years been accused of “meddling” in government affairs. He is reputed to have sent frequent letters to ministers on a range of issues that concern him, but their contents have rarely leaked into the public domain. These letters are known informally as the “black spider memos” because of Charles’s scrawled handwriting.

Backed by the cabinet, Grieve argued that disclosing the letters could create constitutional problems as the public could come to think that the prince had disagreed with government policies.

Grieve said it was crucial that, under the British constitution, the monarch was not seen to be biased towards any political party, or to become entangled in political controversies. He maintained that the prince’s ability to carry out his duties as monarch would be undermined if the letters were made public because he would not be able to recover his position of political neutrality.

Ministers have spent at least £275,000 on hiring lawyers to prevent the exposure of the letters, which, according to Grieve, “were in many cases particularly frank”.

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The legal battle started in 2005 when the Guardian submitted a freedom of information request for copies of the prince’s correspondence with ministers between September 2004 and April 2005. During those months, the prince exchanged 27 pieces of correspondence with ministers in seven Whitehall departments – and it is these letters that would be published if the Guardian succeeds at the supreme court.

The prince insists he wants to “promote and protect what is good about Britain and its people”, and that this entails him “acting personally as a catalyst to facilitate change, to generate debate, or to raise overlooked issues”.

Issues that have concerned him have included genetically modified crops, planning and the environment. However, Charles insists that he does not stray into party political matters.

The Guardian reported last year that the prince is preparing to reshape the monarch’s role if he becomes king and make “heartfelt interventions” in national life, in contrast to the Queen’s longstanding public silence.

An unauthorised biography of the prince by journalist Catherine Mayer, published in February, said Charles intends to be a more campaigning sovereign than his mother, although she fears that Britain will not be ready for his radical new style of monarchy.

At the hearing lasting two days in November, Lord Neuberger, the president of the supreme court, and six senior judges, heard arguments from barristers for the government and the Guardian.

They scrutinised how three judges in a freedom of information tribunal in 2012 had ordered the publication of the letters, ruling that the public should know how the prince seeks to alter government policy.

“The essential reason is that it will generally be in the overall public interest for there to be transparency as to how and when Prince Charles seeks to influence government,” the three judges in the tribunal said.

Grieve vetoed the tribunal’s decision a month later and abruptly blocked publication of the letters. Last year, the court of appeal, headed by Lord Dyson, the leading civil judge in England and Wales, ruled that Grieve’s veto was unlawful.

Dyson concluded that Grieve had “no good reason for overriding the meticulous decision” of the tribunal and could not overrule its verdict “merely because he disagrees with” it.

The letters at the centre of the dispute involved ministers in the Cabinet Office, and the departments covering business, health, schools, environment, culture and Northern Ireland.