The Foreign Office has upgraded its advice on Kenya, warning against all but essential travel to the country.
Britons already in Kenya are urged to exercise "extreme caution" and avoid all political gatherings and large public meetings for fear of unrest.
More than 300 people have been killed in violence following the disputed presidential election.
Some 290,000 UK tourists visit Kenya each year and an estimated 7,000 are thought to be there at present.
In total, the Foreign Office estimates that about 30,000 Britons are currently working or holidaying in the country.
Earlier, it warned against non-essential travel to the capital Nairobi and Mombasa.
Now its upgraded advice says: "There is uncertainty about the security and political situation in the country especially in light of the mass rallies planned for 3 January and continued unrest linked to the disputed presidential election result.
"If you are currently in Kenya you should exercise extreme caution and seek advice locally either from your tour operator or the local authorities particularly if you need to travel."
On Wednesday, Gordon Brown pledged the UK's full backing for a fresh diplomatic drive to end the crisis.
Last week, the Foreign Office issued a warning against non-essential travel to Pakistan following the assassination of Benazir Bhutto.
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