Four British nationals, including two children, have been injured in the bomb attack on the Marriott hotel in the Pakistani capital Islamabad.
They are being treated in hospital in the city, the Foreign Office said.
Their condition is not known.
The UK Foreign Secretary, David Miliband, condemned the bombing as "yet another shocking and disgraceful attack without justification".
Dozens were killed after a suspected suicide bomber detonated a car bomb.
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Pakistani troops kill 35 militants in clash-military ... Miliband said the attack would reinforce Britain's determination to combat "violent extremism" in Pakistan.
"Such an indiscriminate and brutal act of terror deserves the condemnation of the entire international community," he said.
"We continue to stand shoulder to shoulder with the government of Pakistan against the violent extremists who have no answers but only offer death and mayhem."
Conservative leader David Cameron, who recently visited Pakistan, also condemned the attack.
"Like the attempt on the life of Prime Minister Gilani, which took place during my own visit to Islamabad earlier this month, this is an appalling reminder of the constant threat that terrorism poses to Pakistan, and to its return to democracy," he said.
"It is no coincidence that this attack occurred on the day that President Zardari made his first speech to Pakistan's Parliament.
"Today's attack underlines why Britain and Pakistan must work together at every level to defeat terrorism, and to send a clear message to the terrorists that we will remain steadfast in the face of the threat from terrorism."
The Foreign Office says it is still trying to verify the nationalities of others caught up in the blast.
It has reissued its advice that there is a high threat of terrorism and sectarian violence throughout Pakistan, and says travellers should follow news bulletins and check the travel advice on its website.
(BBC)
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