The UK's highest court is being urged to overturn a ruling which paved the way for letters written by the Prince of Wales to government ministers to be revealed to the public.
Seven justices at the Supreme Court in London are hearing the latest round of a lengthy legal dispute over disclosure of the royal correspondence.
The Attorney General, the Government's principal legal adviser, is challenging a decision by three Court of Appeal judges earlier this year that he has unlawfully prevented the public seeing the letters.
In March they unanimously ruled that he has "no good reason" for using his ministerial veto and overriding the decision of an independent tribunal, chaired by a High Court judge, in favour of disclosure.
But, at the start of a two-day hearing today, James Eadie QC, for the Attorney General, argued that the appeal judges had "erred" in reaching their conclusion.
In 2005 Guardian journalist Rob Evans applied to see a number of written communications between Charles and various government ministers between September 2004 and April 2005.
Mr Eadie told the justices that "the Government departments considered that they had no duty to disclose the requested information, and indeed had countervailing duties of confidentiality and as data controllers not to disclose it", and the Information Commissioner agreed.
The Attorney General had "strong grounds for his opinion that the Government and the Commissioner were right to find that the disputed information was exempt".
Mr Eadie said: "At its heart, the public interest in disclosure was a general one, essentially based on the interest in increasing public understanding and debate regarding the constitutional role of the heir to the throne and his interactions with Government...
"The Attorney General does not under-estimate the importance of that public interest.
"But the general nature of it is such that it seems inconceivable that, in a breach of confidence action, the court would be persuaded that acting in breach of its duty of confidentiality was a fair and proportionate way for the Government to pursue that public interest."
The QC told the panel of justices, led by Supreme Court president Lord Neuberger: "Everyone has the right to respect for their correspondence. Such respect is necessary not only as an aspect of privacy, but also to enable freedom of expression, which would inevitably be inhibited by the removal of the right to communicate privately.
"All the more so in the case of the Prince of Wales, whose freedom to express himself publicly is constrained by his role as heir to the throne."
The case is believed to mark the first time that anyone has challenged the Attorney General's powers to block access to information.
Charles is known for his strong opinions on a range of topics from the environment and farming to complementary medicine and architecture.
He has faced accusations in the past of "meddling" in day-to-day politics and criticism over his "black spider memos" - the name given to the hand-written letters he pens to government ministers expressing his views.
Mr Evans sought disclosure under the Freedom of Information Act 2000 (FOIA), and under the Environmental Information Regulations 2004.
The Upper Tribunal (UT) declared in September 2012 that he was entitled to see ''advocacy correspondence'', described as letters the Prince had written seeking to advance the work of charities or to promote views.
The seven departments Charles wrote to are Business, Innovation and Skills; Health; Children, Schools and Families; Environment, Food and Rural Affairs; Culture, Media and Sport; the Northern Ireland Office and the Cabinet Office.
A month after the UT ruling, the Attorney General used his ministerial veto by issuing a certificate under Section 53 of the FOIA.
He said the public could interpret the letters sent to ministers in the last Labour government as showing Charles to be ''disagreeing with government policy''.
The Attorney General said any perception that Charles had disagreed with Tony Blair's government "would be seriously damaging to his role as future monarch because, if he forfeits his position of political neutrality as heir to the throne, he cannot easily recover it when he is king''.
In his opinion, Government departments were legally entitled to refuse disclosure because the correspondence was undertaken as part of the Prince's ''preparation for becoming king''.
Mr Evans accused him of failing to show ''reasonable grounds'' for blocking disclosure.
The journalist lost his case in the High Court, but Master of the Rolls Lord Dyson, Lord Justice Richards and Lord Justice Pitchford all agreed at the Court of Appeal that the Attorney General had gone wrong in law and the Section 53 certificate should be quashed.
Lord Dyson ruled: ''I do not consider that it is reasonable for an accountable person (the Attorney General) to issue a Section 53(2) certificate merely because he disagrees with the decision of the tribunal. Something more is required.
"Examples of what would suffice are that there has been a material change of circumstances since the tribunal decision or that the decision of the tribunal was demonstrably flawed in fact or in law.''
The Attorney General's actions were also incompatible with European law, said the judge.
Lord Neuberger, Lady Hale, Lord Mance, Lord Kerr, Lord Wilson, Lord Reed and Lord Hughes will decide a number of issues, including the "correct construction" of Section 53 (2) of the Freedom of Information Act, in particular "what is the proper test for determining whether the 'accountable person' has 'reasonable grounds' which entitle him to issue a certificate overriding a decision of the Upper Tribunal".
The court will also look at "whether the decision to exercise the power was lawful and/or taken on reasonable grounds".
At the time of the Court of Appeal ruling the Attorney General was Dominic Grieve. The office is currently held by Jeremy Wright QC.
The UT decided that the Government was required to disclose 27 out of 30 requested items of correspondence.
Mr Eadie said Parliament had "set out a clear and detailed provision which unambiguously enables the accountable person to form his own opinion, and if it differs from the prior decision, to veto that decision".
It was clear it had decided that "the highest level of government should be permitted to have the final say as to whether information, the disclosure of which it considered to be damaging to the public interest, should be disclosed".
He told the justices in written submissions: "The issues on which the Attorney General took a different view from that of the UT are fundamentally constitutional and political ones, on which it is entirely proper for the legislature to decide that the executive should be the primary decision-maker."
He said Parliament "did not seek to exclude judicial review of the ministerial decision to veto", adding: "Thus, those decisions remain subject to judicial review."
The Attorney General's decision in relation to the royal letters was made in consultation, he said, with the Cabinet as well as former ministers.
During the proceedings the justices will hear argument on behalf of Mr Evans on why he says the appeal should be dismissed.
In written submissions before the court his QC, Dinah Rose, says the Attorney General has "failed to show any error of law" by the Upper Tribunal.
It was "not reasonable" for the Attorney General "to maintain an opinion based on arguments which had already been fully considered and rejected by the tribunal, on the basis that the appellant simply disagreed with the tribunal".
The tribunal was an "independent and impartial court", while the Attorney General "is neither independent or impartial, but is a member of the Government which is seeking to prevent disclosure of the documents".
Ms Rose argues that "no factors capable of amounting to reasonable grounds" had been advanced, and the Attorney General "has failed to show any reasonable grounds for his decision".
The hearing continues tomorrow. The judges are expected to give their decision at a later date.