David Cameron is cutting short his visit to Balmoral in order to chair an emergency Cobra meeting over the ongoing siege in the Westgate mall in the Kenyan capital Nairobi.
The emergency response committee Cobra has already held two meetings over the attack, in which three Britons have been confirmed dead.
The prime minister wrote in his Twitter account: "I'm cutting short a visit to Balmoral to return to Downing St to chair Cobra late this afternoon, dealing with the Kenya terror attacks."
A team of Britons has been sent to Nairobi to offer consular services. British diplomatic staff from neighbouring Ethiopia have also been drafted in.
The British high commissioner in Nairobi said his crew was helping the affected Britons.
Cameron had earlier called the attack "an absolutely sickening and despicable attack of appalling brutality".
He also warned that the number of British casualties is likely to rise. "Because the situation is ongoing we should prepare ourselves for further bad news," he said.
Citizens from France, Canada and China have also been confirmed dead in the al-Shabaab-led attack.