Former British Prime Minister Harold Wilson may have decided to quit office because he was suffering the early stages of Alzheimer's disease, a new research has suggested.

At the time he said he was simply mentally and physically exhausted, prompting an array of conspiracy theories seeking the real reason for his departure days after his 60th birthday in 1976.

But according to The Independent, an analysis of his speech patterns has found that Wilson may have already been suffering the early stages of the disease which went on to destroy his prodigious memory and powers of concentration.

Dr Peter Garrard, Reader in Neurology at the University of Southampton School of Medicine, reports he has found a decline in mental function in Wilson's final months in Downing Street. He concludes that the onset of the disease could have contributed to Wilson's shock decision to step down as Prime Minister and Labour leader.

His research suggests that during his final months in office Wilson was losing his distinctive voice - seen as an indication the disease was beginning to have an impact on his speech patterns.

Dr Garrard's analysis have been published in the Journal of Neuro-linguistics.

Alzheimer's affects more than 400,000 Britons, and is the most common type of dementia. As it develops, the structure of the brain is attacked and the transmission of messages between different parts of the brain is impaired by a shortage of the right chemicals.