Supreme Court due to rule on Prince Charles letters
The Supreme Court is set to rule on whether Prince Charles's letters to government should be made public.
The UK's highest court has been asked to judge whether the Attorney General's office acted unlawfully when it prevented their publication in 2012.
The Guardian newspaper is seeking the disclosure of the letters written to government departments between 2004-5.
It has been argued that releasing the so-called "black spider memos" would undermine the prince.
The Guardian said it had been "pressing the government" for 10 years to see the letters, written by the Prince of Wales to seven government departments.
He is heir to the throne and, as the royal family's own website explains, it is central to the British constitution that the reigning monarch should remain politically neutral.
The former Attorney General Dominic Grieve has said that any perception the prince had disagreed with the then Labour government in 2004-5 "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".
By BBC royal correspondent Peter Hunt
After 10 years, several court cases, and a cost to the taxpayer of hundreds of thousands of pounds, an end appears to be in sight for this dogged pursuit of Prince Charles's black spider memos.
A final resolution will only be delayed if there's a referral on an issue of European law to the Court of Justice in Luxembourg.
If not, the prince's private letters will either remain confidential, as the sender intended, or they'll be exposed to a large and inquisitive audience.
His friends say he has a right and a duty to communicate with the government of the day.
His critics accuse him of meddling and argue the way he operates should be transparent.
One day, as things stand, he'll be king.
As that day draws closer, the influence Prince Charles attempts to exert on ministers - and attempt he does - has greater relevance and significance.
Guardian journalist Rob Evans originally applied to see letters under the Freedom of Information Act 2000 and the Environmental Information Regulations 2004, but was initially denied by the information commissioner.
But several legal decisions followed:
- In September 2012 the Upper Tribunal ruled that Mr Evans and the public were entitled to see "advocacy correspondence" from the prince (27 out of the 30 letters)
- The seven government departments concerned did not appeal but Dominic Grieve, then attorney general, imposed a veto
- In March 2014, three Court of Appeal judges unanimously ruled that Mr Grieve had "no good reason" for using the ministerial veto
Seven justices at the Supreme Court in London have now heard a challenge by current Attorney General Jeremy Wright QC against the Court of Appeal ruling.