The letter, from Lord McGregor, chairman of the Press Complaints Commission to Sir David, whose report, leaked last weekend, calls for statutory controls on the media, indicates that John Major, Lord Mackay, the Lord Chancellor, Lord Wakeham, the leader of the Lords, and Kenneth Baker, the former Home Secretary, knew that the royal couple were responsible for leaks which many politicians have cited as evidence that new laws banning bugging and telephone tapping are required.
Last night, Lord McGregor verified the contents of the letter dated 11 December 1992 and criticised its leak to the Guardian.
The letter discloses that Lord Rothermere, owner of the Daily Mail and the Mail on Sunday, warned Lord McGregor that the Prince and Princess had 'recruited national newspapers to carry their own accounts of their marital rifts'.
It also reveals that Sir Robert Fellowes, the Queen's private secretary, assured the commission that that the Princess had not been party to leaks to the press, prompting Lord McGregor to accuse the media of an 'odious exhibition of journalists dabbling their fingers in the stuff of other people's souls'.
Less than 24 hours after the statement was made, however, Andrew Knight, executive chairman of News International, told Lord McGregor that the Princess 'was participating in the provision of information for tabloid editors', the letter says.
'At the first opportunity, I spoke to the Lord Chancellor and to Lord Wakeham (Leader of the House of Lords). Lord Wakeham invited me to repeat what I had told him to one of the Prime Minister's private secretaries, and subsequently spoke to Robert Fellowes, who was then in Paris with the Queen. Sir Robert Fellowes later spoke to me in order to apologise for what had happened and to say that his assurance had been given in good faith.
'I told ministers that, if the commission were criticised for a failure to deal effectively with the reporting by the press of the publication of Mr Morton's book and related matters affecting the Royal Family, I should be prepared to issue a public statement containing a narrative of events.'
Sir Robert later apologised, saying that his comment had been in good faith.
The letter accuses the Princess of Wales of seriously embarrassing the commission. Her actions, it says, 'undermined the purpose of this carefully timed and emotively phrased statement'.
Lord McGregor continued: 'Against this background and the likelihood that information would continue to be leaked to the press about the personal relations of the spouses by their partisans, I concluded that the commission had no alternative but to hold their hand for the time being.
'I therefore urged upon Mr Anson and Sir Robert Fellowes the desirability of their sending as formal complaints to the commission a selection of the large number of alleged factual inaccuracies about the Royal Family, taken from the press, which they had sent to me.'
Clive Soley, the Labour MP whose Bill to force the press to correct factual inaccuracies is due its Second Reading in the Commons this month said that if the report was true, 'this doesn't do the Royal Family any good'.
It showed the danger of a tribunal enforcing a statutory code with heavy fines on the press when the rift they were reporting would have been shown to be factually accurate in the end.