Mr Lowther-Pinkerton addressed his letter to Hamish Mykura, head of history, science and religion at Channel 4, but Mr Lygo said it was more appropriate that he respond.

Our unprecedented decision to offer the opportunity for you and Mr (Paddy) Harverson, as the princes' representatives, to view the film was taken in the light of the press reports about this programme in which it was falsely claimed that it contained explicit and distressing images of the crash in which Diana, Princess of Wales died.

We felt that, given the obvious potential distress to the princes if the content of the film was as reported, it was incumbent upon us to allow this opportunity for you to satisfy yourselves on behalf of the princes that the film contains no images of the dying princess and so that they could be accurately briefed on its actual contents.

Accordingly, the opportunity to preview the programme was made as a courtesy in the highly unusual circumstances created by the misleading press coverage and, therefore, your requests concerning the editorial content of the programme were unexpected.

We are satisfied that the programme goes no further, visually or in tone, than other British television programmes, including BBC documentaries and UK newspaper reports Princes' letter to C4

In commissioning this programme we absolutely did not intend to cause distress to the princes. We unequivocally regret that distress has been caused to them by the press coverage and that the very broadcast of this film will cause further distress.

However, Channel 4 must weigh these understandable emotions against the important public value which we believe there is in this documentary which we still believe to be a responsible film which sheds important light on a crucial aspect of the car crash and its immediate aftermath through the use of a limited number of non-explicit photographs and eyewitness accounts of the aftermath of the crash.

We do not believe that this programme is in any way disrespectful to the memory of Diana, Princess of Wales.

'First-hand testimony'

You question the public interest of the documentary.

We believe that, in the context of a measured and responsible history programme, these photographs provide, for the first time, an accurate and detailed eyewitness record of an event of international importance that for ten years has been obscured by conspiracy theories, claims and -counter-claims.

They support the first-hand testimony of passers-by and the photographers at the scene who have been publicly criticised and condemned for their behaviour that night.

You ask us 'not to broadcast any photographs which depict the crashed car whilst the princes' mother lies dying in its wreckage'.

What you are asking us to do would create a dangerous precedent in that it would have a serious impact on the legitimate and important on-going coverage by news programmes and in documentaries of high profile tragedies

In addition, you ask us to remove the shot of the ambulance in which the princess was taken to hospital and in which she was given emergency treatment. In none of these photographs, almost all of which are black and white and grainy, is the princess in any way visible.

We are satisfied that the programme goes no further, visually or in tone, than other British television programmes, including BBC documentaries and UK newspaper reports.

In particular, the programme's producer, Philip Armstrong-Dampier confirms that the three colour pictures from inside the tunnel, taken by a passer-by, Mike Walker, were used in a BBC Panorama documentary in the week following the accident and subsequently in another BBC documentary, Diana: The Conspiracy Files in December 2006 and were published in the Sunday Times and one was used in a recent Channel Five documentary.

I understand the photograph showing the ambulance has been commercially available from reputable picture agencies since 1997 and was used in the Panorama report in 1997 and numerous TV and newspaper reports subsequently.

'Dangerous precedent'

By making this request, you are in effect asking us on behalf of the princes not to broadcast this programme at all since these photographs are crucial to the programme's evidential arguments and analysis.

Furthermore, what you are asking us to do would create a dangerous precedent in that it would have a serious impact on the legitimate and important on-going coverage by news programmes and in documentaries of high profile tragedies like 9/11 and the 717 bombings, or accidents in which public figures have died.

In the only photograph which shows any aspect of the car's interior the occupants have been completely obscured. This picture appears in the same form as it was published by the Sun newspaper on its front page on 14 July 2006.

As you know, the photograph shows the French doctor, Frederic Mailliez leaning into the car to administer oxygen. Dr Mailliez's own testimony is a significant part of the film.

He is explicit that he was not directly obstructed by the photographers, as was widely reported at the time and since, when giving first aid and the picture provides the clear evidence which corroborates his story.

Channel 4 has a proud track record of responsible journalism in respect of our coverage of these events, which has been strongly in the public interest.

We were the first major media outlet comprehensively to deconstruct many of the myths and conspiracies propagated by Mr Fayed and his advisers in our current affairs series Dispatches called The Accident, shown on 4 June 1998, in which we presented crucial evidence pointing to the fact that the crash was in fact a tragic accident.

Subsequent programmes have revealed further evidence to support this.

You will have seen from reports later in the week and over the weekend that journalists who have watched the film have taken an entirely different view and agree that it is an important and responsible documentary.

We firmly believe this view will be shared by those who choose to watch it.