Diana, Princess of Wales was killed because she planned to expose senior members of the British arms trade involved with land mines, a leading lawyer claimed yesterday.

Michael Mansfield QC, who represented Mohamed al-Fayed in the inquest into the death of his son Dodi and the former royal, said Diana claimed she had an "exposure diary" in which she was going to unmask the people most closely involved with the British manufacturing of land mines.

He told the Hay Festival in mid Wales: "I think everyone remembers she raised the profile of the land mines.

"Everybody is aware that the British involvement in the arms trade, particularly land mines, is and was a huge vested interest.

"It seems to me she had planned various visits. She had already been to Angola, she was going to Cambodia later in the year.

"She was going to set up an institute for the victims of the land mines that had been exploded."

He added: "A large number of land mines had been manufactured by the British and I think, and a witness who knew her well claimed, that she had an exposure diary in which she was going to expose the people most closely involved in the British arms trade. It seems to me that is not unrelated (to her death)."

Mr Mansfield said there is a missing box of papers which could contain crucial information.

He said: "Nobody really knows what was in it. The box exists but when it was opened there was nothing in it and everybody has forgotten what was in it.

"I don't know what was in it. It is said there were papers in there, it may have been the diary or notebook she was keeping in relation to the arms trade or it may be other correspondence between the royal family and herself."

Mr Mansfield said he believed the car crash in Paris in 1997 was "more than a mere accident" when he first heard about it.

He said: "Two people so vilified suddenly end up in a crash. I started to ask questions about who does this benefit, how did this come about?

"I got asked to do the case from a different channel and they didn't know I was already very interested in the opportunity, which very few of us ever get, to get senior security service chiefs and senior politicians in a witness box.

"I felt very strongly there was more to this case than a mere accident." He added: "The verdict of the jury was not accidental death. The jury had that option and chose not to take it, they came back to unlawful killing contributed to by the paparazzi and following vehicles.

"The interesting thing is most immediate vehicles were driven not by paparazzi but people they have never managed to trace."

When asked how he distanced himself from conspiracy theorists, Mr Mansfield said: "I think most people think I'm a lunatic and that's fine.

"I'm not a conspiracy theorist about everything and there is something badly botched as opposed to conspiracy but it's a very healthy analysis. It gets you to ask questions you wouldn't otherwise ask."

Mr Mansfield also represented Barry George during his trial for the murder of television presenter Jill Dando.

Speaking about the case, he said pressure to find a suspect means mistakes are often made.

He said: "There is a propensity in the system as a whole to find a body, when it's such a high profile murder you've got to find somebody.

"In the end the propensity goes as far as to say somebody has got to be paying for this crime.

"The system is under the most pressure when the miscarriages occur because people are not looking carefully enough."

He said he was stunned when Mr George, who has since been cleared of the crime, was convicted.

He said: "Those are the cases when I have to walk home. I was so angry. I thought I would hit somebody if I stayed around."

Mr Mansfield said there was no way Mr George could have been guilty.

He said: "The window of opportunity meant whoever did it had to be waiting outside her doorstep to be able to shoot her dead within a matter of seconds of her arrival and nobody saw this loner Barry George waiting.

"He didn't have a car so he could not even have been waiting in a car.

"It was a plainly professional job done very quickly, with people disappearing like sand through the hand.

"It had to be someone who knew what they were doing and Barry George didn't fit that at all."

Mr Mansfield also represented the families of the victims shot dead by British soldiers in Londonderry in the Bloody Sunday massacre.

He pre-empted possible criticism into the report of the Bloody Sunday Inquiry, which is due to be published in two weeks, saying the report was vital for the families.

He said: "I want to pre-empt what is almost certain to happen, which is a welter of criticism that it cost too much money and took too long and what was the point?

"For the families I have represented it means an incredible amount and in my book I describe the effect on one of the families I represented watching as a soldier, under gentle cross examination, who finally admitted it was his trigger and his rifle that shot dead this particular civilian.

"His wife was sitting in the gallery in Westminster with their children. She collapsed and was carried out.

"All she said was: "Thank you, I don't want to come back again, all I've waited for is to hear that admission."

He said the report was a vital part of the peace process, adding: "One of the bricks in the wall was this inquiry. It is the legacy of British involvement.

He added: "What we are all looking for is not just who pulled the trigger, there have been films and books about that, it is how far up the chain of command, military and political, the authorisation went, because it's important for our society to ensure blind eyes are not turned again and this situation does not occur again."