Fertility specialist, Panayiotis Zavos, told a news conference in London on Saturday that the first embryo was implanted two weeks ago, but he cautioned that the woman, an unnamed 35-year-old, might not become pregnant.
"We are waiting for the results this weekend. We expect success but it could result in no pregnancy," he said.
Dr Zavos said he was in London to launch an international search for women willing to be the surrogate mothers of cloned human embryos.
His shock announcement was immediately denounced by the British government, which noted the practice was illegal in Britain, and drew condemnation from the Vatican.
"We made a manifesto commitment to prevent this happening in the UK and we acted swiftly to deliver this commitment by passing the Human Reproductive Cloning Act at the end of 2001," Health Secretary John Reid said.
"This explicitly bans any attempt to create a cloned human baby in the UK. We are also working to achieve a worldwide ban on reproductive cloning through the United Nations."
It also drew scepticism from other quarters.
"This is not the first time Doctor Zavos has made claims without producing any form of evidence to substantiate them," Patrick Cusworth of anti-abortion charity LIFE told reporters.
"This is not the first time Doctor Zavos has made claims without producing any form of evidence to substantiate them. LIFE as an organisation would probably greet Doctor Zavos' latest claim with a certain amount of scepticism."
Anti-abortion charity LIFE.
"LIFE as an organisation would probably greet Doctor Zavos' latest claim with a certain amount of scepticism.
"However if what Dr Zavos says turns out to be true, I would say that he has exposed this 35-year-old woman's vulnerability to almost incredible risk," he added.
On 13 May last year, Zavos, who is based in Lexington, Kentucky, said he had obtained a human embryo through cloning and he intended to implant it soon afterwards.
He said the clone was made using cells taken from a 45-year-old American woman in a secret laboratory outside the United States, in a country he would not name.
At the time, he said he would not implant the embryo until exhaustive checks had been done.
"We are not that desperate to transfer embryos without knowing the complete story about its genetic stability and its biological status and its ability to give rise to a healthy child," he said.
Zavos was previously highly critical of the group Clonaid, which claimed in 2002 to have cloned a human baby.
Clonaid was formed by the controversial Raelian sect. It never produced scientific proof to support its claims and was treated with widespread scepticism by experts.