The Attorney General's decision to block public disclosure of letters the Prince of Wales wrote to Government ministers has been upheld by three High Court judges.
Guardian newspaper journalist Rob Evans accused Dominic Grieve of going wrong in law and "overriding an independent and impartial tribunal''.
But the Lord Chief Justice, Lord Judge, sitting with Lord Justice Davis and Mr Justice Globe, refused to overturn Mr Grieve's veto last October on the release of correspondence between Charles and seven Government departments.
They ruled that the use of the veto was lawful, and that Mr Grieve had reasonable grounds for deciding it was "an exceptional case meriting use of the ministerial veto to prevent disclosure and to safeguard the public interest".
Mr Grieve, the Government's principal legal adviser, said his decision was based on his view that the correspondence was undertaken as part of the Prince's ''preparation for becoming king''. Making the letters public could potentially damage the principle of the heir to the throne being politically neutral, and so undermine his ability to fulfill his duties when king, said Mr Grieve.
The Government had been ordered to disclose letters sent by Charles during a seven-month period between 2004 and 2005 after Mr Evans won a freedom of information appeal.
Three judges at a freedom of information tribunal allowed Mr Evans's challenge against the Information Commissioner, who had upheld refusals by the Government departments to release the correspondence.
The tribunal judges ruled that Mr Evans was entitled to ''advocacy correspondence'' written by Charles, described as letters he had written seeking to advance the work of charities or to promote views. But Mr Grieve issued a certificate vetoing disclosure under section 53 of the Freedom of Information Act 2000 and under the Environmental Information Regulations 2004.
Dinah Rose QC, appearing for Mr Evans, argued at the High Court the use of the veto was legally flawed and the Attorney General had ''adopted an impermissible blanket approach, unreasonably failing to consider the public interest balance in respect of each individual piece of correspondence''.
But Lord Justice Davis said the reasons given by Mr Grieve were "proper, rational and make sense" and he was entitled to consider the prince's correspondence as a whole. . The judge also rejected Ms Rose's argument that section 53 could not lawfully be used in respect of environmental information because ''the exercise by the executive of that power to override the decision of an independent and impartial tribunal'' was incompatible with EU law.