Linda Stewart, a 46-year-old mother of four, slipped on the bank and plunged into the fast-flowing Galana River as she tried to get a better look at a spectacular waterfall in Tsavo National Park. Her husband John, 50, watched in horror - then dived after her.
A third Briton, 18-year-old Christopher Williams, jumped after him to help.
Last night Kenyan officials assumed all three had drowned, although a boat and air search by police and wildlife rangers failed to find their bodies after Saturday's tragedy.
Mr and Mrs Stewart, from Morpeth, Northumberland and married for 29 years, had made the four-hour drive to the park from their holiday hotel on the Kenyan coast. They had flown to Africa for their two-week holiday three days earlier.
British authorities in Kenya said the heroism of the two men probably cost them their lives. The tragic excursion is thought to have been organised by local tour operators.
The Stewarts and their party drove from the coastal resort, Bamburi, to see Lugard's Falls and the local wildlife.
A British High Commission spokesman in the Kenyan capital, Nairobi, said: "We now think the three of them are dead. But we cannot say for definite what has happened to them until they are found."
A Foreign Office spokesman said Mr Williams lived near the Stewarts' holiday resort with his parents.
Mrs Stewart's parents James and Doreen Gordon, who lived near the couple in Morpeth, have sat by the phone waiting anxiously for news.
Linda's sister Doreen Harrison said: "They were a lovely couple - you would never believe they had been married for so long. They used to walk hand in hand in the park and canoodle like a couple of newlyweds."Reuse content