Mishcon de Reya has threatened legal action against Mirror Television over its attempts to make a film called "People's Princess" without permission from Diana's estate.
But Kelvin MacKenzie, Mirror Television's managing director and executive producer of the film, said the project would go ahead and described the lawyers' move as "outrageous".
It is the first show of strength by Mishcon after its announcement this week that it had applied for trademark rights to images of the Princess, on behalf of Diana's estate and her Memorial Fund. The firm is also attempting to secure rights on the words "Diana, Princess of Wales". It says it is attempting to stop any future misuse, exploitation or commercialisation.
The lawyers said in a letter to the producers: "It has been brought to our attention that you intend to produce a film entitled `People's Princess'.
"No one has sought permission from Kensington Palace, the Spencer family or ourselves.
"You will appreciate that a film like this, produced so soon after the untimely death of both the Princess and Dodi Al Fayed, is entirely inappropriate and insensitive not only to their memories but also to their families."
The planned pounds 1.5m film is a production for L!VE TV, whose programmes include topless darts, the "News Bunny" the weather in Norwegian.
The film portrays Diana's search for personal happiness after the break- up of her marriage and fulfilment in her public role, despite harassment by the paparazzi. It ends as a Mercedes enters the Paris underpass.
Mr MacKenzie said the film would be a "tribute" to the late Princess and would be a story that "should and will be told".Reuse content