A statement from Charles' office said he "believes, as have successive governments, that he should have a right to communicate privately". It added: "The publication of private letters can only inhibit his ability to express the concerns and suggestions which have been put to him in the course of his travels and meetings."
The laws were tightened in 2010 to exempt communications from the Queen, Prince Charles and Prince William - for 20 years while the person is alive or five years after the individual's death.
Princely views ...
On the armed forces
Writing about the delayed replacement of military aircraft, he said: "I fear that this is just one more example of where our Armed Forces are being asked to do an extremely challenging job (particularly in Iraq) without the necessary resources."
- To then Prime Minister Tony Blair, 2004
On farming
"So much depends on the consumer demanding British produce and I only wish that more could be done to encourage people to buy British ... it would be splendid if the Government could find innovative ways to give the necessary lead."
- To Blair, 2004
On badger culling
"I do urge you to look again at introducing a proper cull of badgers where it is necessary. I, for one, cannot understand how the 'badger lobby' seem to mind not at all about the slaughter of thousands of expensive cattle, and yet object to a managed cull of an over-population of badgers - to me, this is intellectually dishonest."
- To Blair, 2005
On illegal fishing
"I particularly hope that the illegal fishing of the Patagonian Toothfish will be high on your list of priorities because until that trade is stopped, there is little hope for the poor old albatross, for which I shall continue to campaign."
- To the Fisheries Minister, 2004
On preserving heritage
"I think you will know by now - to your cost! - that these are matters about which I care deeply - chiefly because I have witnessed so many failed opportunities to create imaginative, and innovative heritage-led regeneration initiatives ... "
- To then Health Secretary John Reid, 2005, about redeveloping an old hospital.
- Washington Post-Bloomberg, AP