The researchers suggest they may be able to develop treatments for children
Scientists have revealed the earliest developmental changes that lead to Down's syndrome.
The team from Barts and the Royal London say the changes to embryonic stem cells are caused by the presence of an extra copy of chromosome 21.
The study, in the American Journal of Human Genetics, says the extra chromosome sets off a chain of genetic changes in the developing embryo.
The Down's Syndrome Association welcomed the "excellent" research.
Down syndrome belongs to a group of conditions called "aneuploidies", which are defined by an abnormal loss or gain of genetic material, such as fragments of chromosomes or whole chromosomes.
Aneuploidies cause congenital anomalies that are a prime cause of infant death in Europe and the US, and are currently on the increase with advancing maternal age in European countries.
Around one in every 1,000 babies born in the UK will have Down's syndrome.
There are 60,000 people in the UK with the condition.
The international team of researchers, which also included scientists from the US, Australia, Spain and Switzerland, looked at embryonic stem cells from mice which had been genetically engineered to carry a copy of human chromosome 21.
They discovered that the presence of the extra chromosome 21, known as trisomy 21, disturbs a key regulating gene called REST, which then disturbs the cascade of other genes that control normal development at the embryonic stem cell stage.
The scientists also found that one gene (DYRK1A) which is present on chromosome 21, acts as the trigger for this disturbance.
Dean Nizetic, professor of cellular and molecular biology at Barts and the London, said the work could one day lead to molecule-based therapies which could alleviate the effects of Down's syndrome.
"We hope that further research might lead to clues for the design of new therapeutic approaches tackling developmental delay, mental retardation, ageing and regeneration of brain cells, and Alzheimer's disease.
He said he believed the genetic effects continue throughout life.
"I suspect that it's not just important for the development of brain cells but for their maintenance throughout life; how cells age and how they can cope with stress.
"That's an area that could be approached with regard to therapies."
Professor Nizetic suggested future research should be directed into basic molecular mechanisms that could one day develop into treatments to children with Down's syndrome in the first few years of life when the brains are "plastic" and rapidly developing.
And he said that the same areas of the human genome have been thought to play a part in Alzheimer's disease - so research could also lead to treatments for that condition.
Carol Boys, chief executive for the Down's Syndrome Association said: "Any research that helps us to understand more about some of the complex medical conditions that are commonly associated with Down's syndrome can only be a positive step forward.
"The development of therapeutic treatments for these sometimes complicated health problems that can be associated with the condition will hopefully lead to an improvement in the overall health of people of with Down's syndrome."
She added: "We understand that research is slow, but the initial results look extremely positive and we look forward to the continuation of the excellent work of this dedicated research team with interest."