And, he says in a newspaper article, those who voluntarily bring their private lives into the public arena "may place themselves beyond the PCC's protection. And must bear the consequences of their actions".
Privately the ex-Cabinet minister believes the Princess's Panorama interview, due to be screened on BBC1 tomorrow night, is a mistake which will rebound on the Royal Family. In today's Mail on Sunday, Lord Wakeham argues that the Royal Family is not immune from "the rules that govern us all". He adds : "If we court publicity, or reveal matters that should remain private, we run the risk of finding ourselves in a spotlight which may prove harsh and intrusive."
His entry into the row marks a new chapter in the strained relations between the commission and the Royal Family.
Last month, at a private dinner, the Princess urged Lord Wakeham to take a tougher line against press invasions of privacy. But over the meal, which was also attended by Sir Tim Bell and Gordon Reece - leading figures in the advertising and media worlds - the commission chairman rejected Princess Diana's arguments.
He is privately critical of the Princess for reawakening the controversy over her marriage. Lord Wakeham believes that, while Prince Charles's documentary series with Jonathan Dimbleby was ill-advised, it was an understandable riposte.
Princess Diana had already collaborated with the journalist Andrew Morton to present her side of the royal separation.
Meanwhile the Prince of Wales yesterday said the Queen Mother was on her feet again and "in wonderful spirits", after spending more than half an hour with her.
The Prince, the first visitor his grandmother has had since her hip replacement operation last Thursday, said she was determined to exercise her hip as advised by doctors. It was her "iron will, mind over matter", he said.
As he left the King Edward VII Hospital for Officers in central London, the Prince said his grandmother was in "very good form, she's in wonderful spirits".
Ian Jack, page 5; Questions for the Princess, page 8; Leading article, page 20