A controversial US fertility specialist told a news conference in London on Saturday that he has implanted a cloned embryo into a woman's womb.
Professor Panos Zavos gave few details and no evidence, and said it was still too early to tell if the procedure had resulted in pregnancy.
He said the embryo came from the immature egg of the infertile 35-year-old woman, and a skin cell from her husband.
Zavos said the chances are about 30 percent and he would announce the result in two to three weeks. He said it was a "very recent event" that did not happen in the United States, the United Kingdom or anywhere else in Europe.
UK Health Secretary John Reid condemned the attempt to create a cloned human baby as a "gross misuse of genetic science."
Fertility experts said if true, it was an "incredibly risky" step to take, and said Zavos was giving false hope to people desperate to have children.
Zavos also claimed the procedure had been filmed, and he would allow DNA testing to check his claims at a later date.
Reproductive cloning takes DNA from the donor and transfers it into an egg which has had its nucleus, and therefore most of its own genetic material, removed.
It is similar to the technology used to create the first cloned sheep, but its use on humans is illegal in the United Kingdom.
The shock announcement has been greeted by the scientific community and government with a mix of scepticism and disgust.
Bob Ward of the Royal Society said: "It is science fiction until Dr Zavos produces evidence. "What is more worrying is, without being sure of any substance to the claims, some infertile couples may have their hopes falsely raised, which is regrettable."