Prince Charles is "perfectly entitled" to voice opinions or privately lobby ministers, the prime minister has said.
The prince believes the diaries were accessed unlawfully
The prince's role is in the spotlight after a court heard he saw himself as a dissident against political consensus.
Critics say the Royal Family should not use its position to pursue its own views or get special access.
Tony Blair said the prince's views were not party political and had often been "helpful". They had not caused "any difficulties for ministers" he added.
Mr Blair said the extent of Prince Charles' interventions had been exaggerated.
"It is completely unreasonable not to expect that he has views or that he transmits them to government ministers but they are not ones that I have ever regarded as party political," he said.
"For example, he will raise issues sometimes to do with the rural community or issues to do with voluntary organisations in the UK, which I actually find perfectly helpful."
Mr Blair added: "I think that Prince Charles does an amazing job for the country.
"If you look at the Prince's Trust, it's probably one of the most successful voluntary sector organisations in the world, never mind in this country, and I think he's perfectly entitled to express his views and personally I find no problem with it at all."
Earlier, Constitutional Affairs Secretary Lord Falconer said it was "appropriate" for the prince to voice his views.
But he warned the prince not to "cross the line" into party politics - something he had so far avoided.
The prince is suing the Mail on Sunday newspaper for breach of copyright and confidentiality for publishing parts of his diaries.
In extracts about the 1997 Hong Kong handover published in the Mail on Sunday in November 2005, the prince described Chinese officials as "appalling old waxworks".
But the case has raised questions about the prince's wider constitutional role.
His former private secretary, Mark Bolland, told the High Court: "He would readily embrace the political aspects of any contentious issue he was interested in...
"He carried it out in a very considered, thoughtful and researched way. He often referred to himself as a 'dissident' working against the prevailing political consensus."
Lord Falconer told BBC Radio 4's Today programme it was legitimate for the prince to use his position to tell ministers he thought they were wrong on certain issues or to voice opinions publicly.
"It is perfectly appropriate for somebody in the Prince of Wales' position to have views about the countryside, about climate change, about big environmental issues, about the armed forces.
"He is engaged in public life in a way that all of us are.
"There is obviously a line where it gets party political and it would be inappropriate for the monarch in those circumstances to cross that particular line but I do not think he does."