Three British nationals were killed in Saturday's attack on a shopping centre in the Kenyan capital Nairobi, the Foreign Office has said.
And it said that number is "likely to rise as further information becomes available".
At least 59 people were killed and 175 injured in the attack, the Kenyan interior minister has said.
Between 10 to 15 attackers and an unknown number of civilians are still in the building.
The next of kin of the dead British nationals have been informed, the Foreign Office said.
Prime Minister David Cameron called the attack "an absolutely sickening and despicable attack of appalling brutality".
The Somali militant group al-Shabab says it carried out the attack on the upmarket Westgate shopping centre in response to Kenyan military operations in Somalia.
Kenyan interior minister Joe Lenku said 1,000 people had managed to escape from the shopping centre.