The citizens were urged to leave the east African country's port city of Mombasa following a series of attacks.
The latest warnings, issued on Tuesday and Wednesday, may further damage Kenya's tourism sector, one that President Uhuru Kenyatta said was "on its knees" after high-profile attacks in resorts along its Indian Ocean coastline.
Explosions in Nairobi and Mombasa killed seven people on May 3-4, and Kenya has blamed a series of gun and grenade attacks countrywide on the al Qaeda-linked Somali group al Shabaab. The militants killed at least 67 people, including many foreigners, in a raid on an upscale Nairobi shopping mall in September.
Britain's Foreign Office said that unless on essential travel, all its citizens should avoid Mombasa and the surrounding area "following recent terrorist attacks and the continuing terrorist threat in the area".
The warning said there was a high threat from "terrorism, including kidnapping", but excluded Mombasa's Moi International Airport and another airport at Diani on the coast.
"If you are currently in an area to which we now advise against all but essential travel you should consider whether you have an essential reason to remain. If not, you should leave the area," a Foreign Office statement said.
The U.S. Embassy in Kenya also warned its citizens of the continued threat of "potential terrorist attacks in the country", citing targets include hotels, nightclubs and malls.
The embassy and other diplomatic missions were beefing up security due to the threat on foreigners in Kenya as well as the recent spate of explosions in Nairobi, it said in a statement.
Kenyan authorities denied knowing of any imminent threat. "There isn't any specific threat that we know about to warrant these advisories," Interior Ministry spokesman Mwenda Njoka told Reuters.
Western diplomats have privately said Kenyan security forces - which receive aid and training from the United States, Britain and Israel among others - are weakened by rivalries between agencies that also hamper intelligence work.
Al Shabaab has said it will carry out attacks in Kenya to demand Kenyan troops withdraw from neighbouring Somalia, something Kenyatta has rejected.
Kenya sent troops into Somalia in October 2011 to pursue the insurgents it blamed for a surge in violence and kidnappings threatening tourism in east Africa's biggest economy.
Kenya rebuked the United States for warning its citizens over travel to the country after the Westgate mall attack, calling the alert "unfriendly" and asking Washington to lift it.
Sam Ikwaye, head of the Kenya Association of Hotel Keepers and Caters, told Reuters the latest warning could hurt tourism, a significant foreign exchange earner, even further.
“We are receiving many calls from our members telling us that the few clients in their hotels are worried and some are even cutting short their stay," Ikwaye said.