At least another 65 people were injured when a huge bomb exploded at the Pearl Continental Hotel in Peshawar on Tuesday.
Unicef's Peshawar district co-ordination officer Sahibzada Anis said the blast wounded the Briton and two others working for the United Nations children's agency - a Somali and a German. Unicef staffer Perseveranda So, 52, from the Philippines, was killed, the agency said.
Unicef executive director Ann Veneman said: "At the time of the bombing, the hotel was housing many humanitarian workers there to provide life-saving assistance to Pakistan's most vulnerable people."
The Foreign Office spokesman said: "We can confirm that one British man has been injured. He is in hospital. We are offering consular assistance."
He added: "The UK condemns unreservedly all such acts of indiscriminate terrorism."
Pakistani officials said suicide attackers in a truck launched the assault, firing guns as they stormed past guards at the hotel and then setting off a huge blast.
The hotel is a favourite place for foreigners and elite Pakistanis to stay and socialise, making it a high-profile target for militants.
The blast is the latest in a string of attacks in Pakistani cities in recent weeks that officials say are revenge for a military offensive against Taliban militants in the Swat Valley.