PRINCESS Diana's frail mother is suffering from an incurable brain disease which has left her struggling to speak or walk.

Frances Shand Kydd's son and daughters have pleaded with her to move from her Scottish retreat and nearer to the Spencer family seat in Althorp, Northamptonshire.

But she insists she will never leave the Isle of Seil, near Oban.

Mrs Shand Kydd, 68, began to deteriorate after Diana's death in August 1997.

Her speech was affected leading to speculation she could be suffering from an alcohol problem, particularly as she was convicted of drink-driving when her daughter was going through her divorce from Prince Charles.

When her condition worsened after a number of falls, she saw a neurosurgeon who carried out a brain scan and confirmed she was suffering from a progressive disease.

It has never been identified but there is speculation that it could be Parkinson's or Alzheimer's.

Mrs Shand Kydd, who has given up smoking and drinking for the sake of her health, caused concern during her appearance at the Old Bailey during the trial of royal butler Paul Burrell in November 2002.

She confirmed she was suffering from a medical condition when she gave evidence but said: 'I have not had a stroke or any other life-threatening disease. I have to confront and accept the limitations of the condition I have.

'It took a while to diagnose but I feel it is my business to keep it to myself. It's nothing to do with anyone else.' Shortly after the Burrell trial collapsed, Mrs Shand Kydd was nearly killed when her green Audi skidded on oil near her home and overturned.

Her son Earl Spencer and Concern: Mrs Shand Kydd at the Burrell trial daughters Lady Sarah McCorquodale and Lady Jane Fellowes have become increasingly worried about her frailty.

Amy Maclaren-Froom, of the Parkinson's Disease Society, said: 'All her symptoms are compatible with the condition.

'People with Parkinson's are often mistaken for being drunk because they tend to stagger or shake and slur their speech.

'People tend to learn to live with it and die of something else.' She added that sufferers tend to have Parkinson's for about ten years before needing fulltime care, although not all develop the full-blown form of the disease.

Hannah Laslett, of the Alzheimer's Society, said: 'It could be Alzheimer's as it can cause a loss of balance and affects speech and memory.

'It is a degenerative disease of the brain and in the later stages, there is a physical decline with loss of mobility and speech.' Princess Diana was Frances Shand Kydd's third daughter, born in 1961 when her mother was married to aristocrat Johnnie Spencer, a former equerry to the Queen and heir to Althorp.

The marriage was tempestuous and punctuated with fierce rows and it was not long before Frances began an affair with wallpaper heir Peter Shand Kydd after meeting him at a dinner party.

She split from her husband and was devastated when he won custody of their children - particularly as her mother Lady Ruth Fermoy gave evidence against her.

Diana's mother married her lover soon after she was divorced but the relationship faltered and she escaped to her Scottish retreat in 1972.

Spencer meanwhile met and married Raine, the Countess of Dartmouth and daughter of novelist Dame Barbara Cartland.

Mrs Shand Kydd has vowed to spend the rest of her life at her bungalow on her beloved Isle of Seil.

She has said: 'I have the greatest friends in the world here and the best view on Earth. I'll be staying here until I die and I'll be buried in Oban.

I've already bought the plot.'