Armed men seized the newlyweds' taxi at an intersection in Cape Town late on Saturday and an hour later they forced out the British man, while the body of the woman was found hours later, police in South Africa said.
"We are aware of the incident. We are providing consular assistance to a British national who was involved, and their family," a foreign office spokesperson said.
A British government source said the woman who was killed was not British.
Officials did not name the couple but they were named in reports as Anni Dewani, 28, and British businessman Shrien Dewani, 31. Reports said she was a Swedish national, of Ugandan-Asian descent.
The BBC quoted members of his family in Bristol, western England, as saying they were shocked and still "trying to take it all in".
The couple married two weeks ago and arrived in Cape Town on Thursday, South African regional tourist authorities said.
Police said they had not yet established the cause of her death.
"An extensive search for the vehicle and the female victim resulted in the discovery of the hijacked vehicle in Lingelethu West, with the female victim's body on the back seat," the statement said.
"An autopsy will later reveal the exact cause of her death."