A suicide attack on a hotel near Mombasa killed 15
All UK flights to and from Kenya have been suspended by the government amid fears of an "imminent" threat of terror attacks on British planes.
A total of 1,200 British tourists are now potentially stranded in the east African country after the suspension started at 2200 BST, the Association of British Travel Agents said.
British Airways has cancelled its daily departure to Kenya and is making arrangements to transfer hundreds of passengers into Tanzania to fly them back to the UK.
The action follows warnings from the US about possible terrorist attacks throughout East Africa and south-east Asia, following Monday's suicide bombings in Saudi Arabia which killed 34 people.
But Kenya has criticised the flight ban as an "extreme" action that played into the hands of any would-be attackers.
The Foreign Office has advised against non-essential travel to Kenya, and advised Britons there to keep a low profile and maintain a high level of vigilance in public places.
About 100,000 Britons holiday each year in Kenya, where in November there was a failed attempt to shoot down a plane carrying Israeli tourists from a holiday resort near Mombasa.
Fifteen people, mostly Kenyans, were killed in a suicide attack on the Israeli-owned Paradise Hotel near Mombasa at the same time as the attempt to shoot down the plane.
The BBC's Daniel Sandford said flights were being suspended, according to Whitehall sources, over concerns about "personnel and weaponry" being in place, although it may not necessarily be an attempt to shoot down a passenger aircraft.
The Department of Transport ordered UK airlines to suspend flights to and from Kenya saying "the threat level to UK civil aviation interests in Kenya has increased to imminent."
Peter Kirk, the assistant director of aviation, said: "Developments in Kenya and other recent events demonstrate the continuing terrorist threat to UK civil aviation generally.
"High levels of vigilance and security awareness are therefore essential."
"What we do know is that there is some information and it is a growth really
of information which enables us to say we are now extremely worried that
something might happen.
"We had to give this warning."Matthew Kabetu, head of Kenya's anti-terrorism unit, told the Associated Press on Thursday an al-Qaeda suspect may have returned to the country.
Fazul Abdullah Mohammed is wanted in connection with the US embassy bombing in Nairobi and Dar es Salaam and the attacks in Mombasa in November last year.
The Kenyan Government has issued his photograph saying he may have slipped back into Kenya or may be in Somalia.
He is believed to be the chief architect of November's bombing of the hotel.
Earlier on Thursday, the United States urged its citizens to postpone non-essential trips to Kenya, due to fears of possible terror attacks by groups linked to al-Qaeda.
The State Department fears an attack could be timed to coincide with Thursday's celebration of the Prophet Muhammad's birthday, "Maulid".
Monday's attacks in the Saudi capital, Riyadh, have been blamed on al-Qaeda by the American and Saudi Governments.
UK Foreign Office minister Mike O'Brien told Channel 4 News it was "difficult to say" whether the Kenya alert was linked to the Riyadh attack.
"What we do know is that there is some information and it is a growth really of information which enables us to say we are now extremely worried that
something might happen. We had to give this warning."
British Airways operates daily services between London and Nairobi and a flight
to Nairobi was due to leave Heathrow airport at 2150 BST on Thursday.
Chartered airline Monarch has also cancelled its weekly Sunday flight.
Kenya Airways is still flying between the UK and Kenya, but other Western airlines are said to be considering their position.
Kenyan security minister Chris Murungaru criticised the British Government.
"The action taken by the British Government was extreme and action like this may make it appear like terrorists are making a moral score, a moral victory," he said.
He said Kenya was taking all necessary security precautions against
But a BA spokesman said: "The safety and security of our customers is always our
first priority and will never be compromised."
An ABTA spokesman advised passengers due to travel to Kenya to contact their travel agents to make alternative arrangements.
About 100 of the British tourists in Kenya are with Thomson Holidays.
A spokeswoman said they were speaking to their clients, liaising with the Foreign Office and the Federation of Tour Operators, and had suspended bookings for the next 10 days.
A UK Foreign Office statement said: "British tourists in Kenya wishing to return home should seek the advice of
their tour operator or the British High Commission in Nairobi.
"People intending to travel to Kenya in the near future should seek advice
from their tour operator or airline".
The Foreign Office has set up an advice helpline on 020 7008 0000