Experts cautioned that finding the causes of autism spectrum disorder – a complex syndrome that has been studied for decades – will not be straightforward, and called the effort misguided and unrealistic.

Kennedy, who has promoted debunked theories suggesting autism is linked to vaccines, said during a cabinet meeting on Thursday that a US research effort will "involve hundreds of scientists from around the world."

"By September, we will know what has caused the autism epidemic and we'll be able to eliminate those exposures," Kennedy said.

Autism diagnoses have increased sharply since 2000, according to government figures, and by 2020 the rate among 8-year-olds reached 2.77%, according to the US Centers for Disease Control and Prevention (CDC).

Scientists attribute at least part of the rise to increased awareness of autism and an expanding definition of the disorder. Researchers have also been investigating environmental factors.

The US National Institutes of Health (NIH), a government agency, spends more than $300m (£230m) per year researching autism.

The NIH lists some possible risk factors including prenatal exposure to pesticides or air pollution, premature birth or low birth weight, maternal health problems and parents conceiving at older ages.

Kennedy did not give details on the research project or how much funding will be devoted to autism research.

Since being sworn in two months ago, the former environmental lawyer has slashed the budget for the Department of Health and Human Services (HHS), which includes the NIH, CDC and other government health organisations that oversee food and drug safety and conduct disease research.

"We're going to look at vaccines, but we're going to look at everything," Kennedy later said during an interview with Fox News about the scope of the undertaking. "Everything is on the table, our food system, our water, our air, different ways of parenting, all the kind of changes that may have triggered this epidemic."

In a statement the Autism Society of America called Kennedy's plan "harmful, misleading, and unrealistic".

"It is neither a chronic illness nor a contagion," the society said.

Christopher Banks, the society's president, questioned whether the research efforts would be transparent and said claims that autism is solely caused by environmental factors were "misleading theories (which) perpetuate harmful stigma, jeopardize public health, and distract from the critical needs of the autism community."

Kennedy has also alarmed some over his hiring of David Geier, who has been described by some as a conspiracy theorist, to research vaccines and autism, and on Thursday Democrats in the US House of Representatives wrote to HHS "to express our urgent concern" over the selection of "a biased and discredited individual".

Geier is a leading vaccine sceptic who was fined by the state of Maryland for practicing medicine without a medical degree or licence and prescribing dangerous treatments to autistic children.

The discredited idea that childhood vaccines are linked to autism first gained mainstream attention after a paper published in 1998 in the medical journal The Lancet by British doctor Andrew Wakefield.

Wakefield was later found to have financial conflicts of interest and the UK's General Medical Council found that he falsified his results. The research paper was retracted.