The office of the Prince of Wales has said he had a right and a duty to communicate privately with the government "on any matter he chooses", after the extent of his private meetings with ministers came under renewed questioning.

An analysis of palace records showing that Prince Charles has held private meetings with cabinet ministers at least 36 times since the 2010 general election was seized on by campaigners for a republic who said it showed he was "a political operator and businessman with direct access to government".

Charles has met the prime minister, David Cameron, seven times and in many cases held meetings with ministers who have responsibility for areas in which he has taken a particular interest, according to the Daily Mail.

Following previous coverage by the Guardian of Charles's private meetings with ministers, campaigners for transparency concerned about royal lobbying have called for the government to provide information about the topics discussed as it does after representatives of outside organisations meet ministers.

Details of the meetings with senior political figures in the Westminster and Cardiff governments, including why they were held and what was discussed, have not been made public, in line with a convention of secrecy about communication between both the Queen and the heir to the throne and government ministers.

In a statement, Clarence House said: "The Prince of Wales has a right, indeed it is his duty, to communicate privately with the government on any matter he chooses, to bring his unique perspective and reflect the many issues people raise with him personally on his extensive engagements around the country."

"Given these broad areas of interest, as well as specific events such as the jubilee and Olympics, it is inevitable that HRH may, at times, see some ministers more than others."

Prince Charles's private office said he receives ministers and officials from a broad range of government departments on a regular basis either in office meetings or through initiatives such as the cross-party Campaign for Youth Social Action, which he will lead, or addressing the issue of illegal trade in wildlife.

It added: "Official meetings, sometimes instigated by ministers, are important to the prince in his role as heir to the throne and ministers from successive governments have found that he brings important insights, perspectives and knowledge built over 40 years of experience in a range of areas aimed at transforming lives and building sustainable communities."

However, activists campaigning for an end to the monarchy seized on the figures. The lobby group Republic tweeted: "We've been pushing this issue for some time. Charles is political operator and businessman with direct access to govt."

The high court ruled last month that the public has no right to read documents that would reveal how Charles has sought to alter government policies. Three judges rejected a legal attempt by the Guardian to force the publication of private letters written by the prince to government ministers.

However, Lord Judge, the lord chief justice of England and Wales, and two other judges have given the Guardian permission to appeal against the decision.

The appeal, which is due to be heard in the court of appeal this year, will be the latest stage in an eight-year battle by the newspaper to view a set of letters written by the prince to ministers in seven government departments over a nine-month period.

The cabinet decided that the letters must remain hidden after concluding that they could undermine the public's perception of the prince's political neutrality.

Dominic Grieve, the attorney general, has said that if the letters were published there was a risk that the heir to the throne would be "viewed by others as disagreeing with government policy".