Prince Charles was tonight facing fresh accusations of meddling in government policy after it emerged that he had written directly to ministers in eight Whitehall departments over the last three years.

The heir to the throne, who has strong views on the environment, farming and architecture, wrote to ministers in departments including the Treasury, Foreign and Commonwealth Office and the education department.

Documents obtained by the Guardian also reveal that his advisers pressed senior cabinet ministers to bring government policy into line with the prince's beliefs on matters including hospital building and the design of ecotowns.

The disclosures will fuel growing concern that the prince is continuing to interfere in political matters when many believe he should remain neutral if he wishes to become king.

Leaks of previous correspondence, known among ministers as "black spider memos" because of the prince's sprawling handwriting style, provoked a backlash among politicians furious that an unelected royal was meddling in the affairs of democratic government.

The fresh evidence of his lobbying was obtained using the Freedom of Information Act, although Whitehall departments refused to release the content of the letters. The Guardian has established that since 2006 Charles wrote to politicians leading eight government departments and his advisers wrote to five.

The departments released correspondence from senior aides who run his architectural charity to Hazel Blears, then secretary of state for communities. They show how his charity urged the government to adopt Charles's favoured approach to the ecotowns initiative.

They also wrote to Patricia Hewitt, who was health secretary, to recommend that all hospital trusts planning new buildings should use the design technique pioneered by Charles's architecture charity.

Separately they pressed Andy Burnham, chief secretary to the Treasury at the time, to consider the findings of a study into sustainable ways of increasing the housing supply "which provides support for the [prince's] Foundation's mission to promote timeless and ecological ways of planning, designing and building".

Chris Huhne, the Liberal Democrat home affairs spokesman, said tonight he was surprised that the prince had taken such an extensive interest in activities across Whitehall. "He has to be very careful to respect the traditional separation between the democratically accountable parts of the constitution and the ceremonial parts. The Prince of Wales is entitled to ask about what is going on but if he is urging a particular point of view, then that's a different matter," Huhne said.

The ex-sports minister Richard Caborn said it was wrong to keep the letters secret. He said: "He is entitled to press his views. I would treat it no differently to anybody else's. If he is making his views known to ministers, they should be in the public domain. He can't have it both ways."

In future, however, it is possible that the government will choose never to disclose the prince's correspondence with ministers. Gordon Brown has ordered a block on the disclosure of correspondence sent to ministers by members of the royal family and todayhis clampdown on attempts to expose the scale of the prince's lobbying was boosted when Christopher Graham, the information commissioner, who adjudicates in secrecy disputes, blocked the release of an earlier set of correspondence which involved Tony Blair.

Ministers said there was "a well-established constitutional doctrine that the heir to the throne has a right and duty to be instructed in the business of government in preparation for the time when he himself will be the sovereign".

The prince's aides have denied that he "bombards" ministers. In a strong rebuttal that the prince meddles in politics, Sir Michael Peat, his principal private secretary, said in 2007: "His royal highness is always very careful to ensure he is not politically contentious or party political, and as far as I am aware even his most ardent critic has never suggested he is."

But the prince has also referred to himself as a "dissident" working against the prevailing political consensus, according to his former press adviser Mark Bolland. In 2001, he complained to Lord Irvine, then lord chancellor, about the Human Rights Act and compensation culture and railed against the "degree to which our lives are becoming ruled by a truly absurd degree of politically correct interference".

Clarence House denied tonight that the prince's household had lobbied the government to stop all disclosures of his letters and defended his right to secrecy, saying his role as a privy counsellor gave him the right to communicate confidentially with ministers on matters that concern him. "It is generally accepted that the heir to the throne should be aware of the business of government and that correspondence between government ministers should be treated as private and confidential on all sides," said a spokesman for the prince.

Whitehall departments which have received personal letters from the Prince of Wales since 2006

Department for Food and Rural Affairs

Department for International Development

HM Treasury

Foreign and Commonwealth Office

Department for Work and Pensions

Department for Education and Skills

Department for Communities

Department for Culture, Media and Sport