A fresh legal attempt to sweep away the secrecy surrounding Prince Charles's behind-the-scenes lobbying has been launched at the court of appeal.

The Guardian is attempting to overturn a veto issued by Dominic Grieve, the attorney general, banning the disclosure of private letters sent by the prince to government ministers that sought to alter official policy.

Grieve has said the 27 pieces of correspondence between the heir to the throne and the ministers over a seven-month period contain the royal's "most deeply held personal views and beliefs" and are "particularly frank".

Grieve, with the backing of the cabinet, has blocked the publication of the letters on the grounds that the public could read the letters and come to believe that the prince had disagreed with the policy of the last Labour government.

Such a risk would be constitutionally damaging as Grieve said the heir would find it difficult to regain his position of political neutrality when he was king.

On Monday, Dinah Rose, the QC for the Guardian, opened the two-day appeal in front of the master of the rolls, Lord Dyson, and two other judges, Lord Justice Richards and Lord Justice Pitchford.

She is arguing that Grieve's use of the veto, backed by an earlier court, is legally unjustified and should be quashed.

She told the court that Grieve had acted unreasonably when he issued the veto in October 2012.

Grieve had, she said, merely repeated arguments that had failed to persuade a freedom of information tribunal that had ruled in favour of the newspaper a month beforehand.

The attorney general had not put forward any new facts or legal points, Rose said, when he used the veto.

She said a key question was "in this case did the attorney general show reasonable grounds for forming his opinion. We submit he did not."

In its verdict in September 2012, the freedom of information tribunal had ruled that the correspondence should be published as "it was in the public interest for there to be transparency as to how and when Prince Charles seeks to influence government".

The appeal that started on Monday is the latest stage of a nine-year battle by the Guardian to obtain copies of the prince's correspondence with seven government departments between September 2004 and April 2005 under the Freedom of Information Act.

It is the first legal attempt of this kind to see the prince's secret correspondence with ministers.

The prince has for many years been accused of using his unelected position to meddle in official policy by lobbying ministers with his views on subjects such as alternative medicine, architecture and the environment.

Critics of the monarchy have argued that the deployment of the veto by Grieve was an "affront to democracy" as it sought to protect Charles from legitimate scrutiny.