A British security worker was among those killed in an attack on a UK embassy vehicle in Afghanistan, the Foreign Secretary has said.
An Afghan national working for the embassy in Kabul was also killed, Philip Hammond said, condemning the strike as "appalling".
A second British member of the security team was injured when the vehicle was targeted by a suicide bomber.
Mr Hammond said: "This morning a British Embassy vehicle was attacked in the Afghan capital, Kabul.
"I am deeply saddened to confirm that a British national civilian security team member and an Afghan national working for the embassy were killed in the incident. A second British member of the security team was injured.
"I condemn this appalling attack on innocent civilians supporting our diplomatic activity. The families of the victims have been informed and my thoughts are with them. The Foreign Office will do everything it can to support them.
"This outrage brings home to us once again the courage and perseverance of the people of Afghanistan and members of the international community who support them, who have lived together through decades of conflict.
"We will not allow such inhumanity to deter us from continuing our partnership with the government of Afghanistan.
"I have nothing but admiration for the staff of the embassy, British and Afghan, who work at great personal risk to help build a better future for Afghanistan."
The Afghan Interior Ministry said four Afghans were killed in the attack, including the driver of the embassy vehicle, and 33 civilians were wounded.
Some reports claimed the attack was staged by a bomber riding a motorcycle, while others suggested a car packed with explosives was rammed into the embassy vehicle.
The explosion caused parts of the city to shake and sent a plume of dust and smoke into the air.
The Taliban were reported to have claimed responsibility.
Photographs shared on social media apparently from the scene showed burning wreckage including a car, debris strewn across the ground and bloodied casualties.
Mustafa Deveci, a journalist in the area, told Sky News the attack took place on a "very busy" road.
He said: "I saw five or six civilian cars were damaged."
Sir Simon Fraser, head of the UK Diplomatic Service, tweeted: "Shocking news from Kabul of attack on British Embassy vehicle. Working to get full facts. Thoughts with all our people."
Franz-Michael Skjold Mellbin, the European Union's special representative in Kabul, wrote on Twitter: "Strongly condemn this morning's indiscriminate attack in Kabul that left many Afghans dead and injured UK colleagues."
It is the latest in a wave of attacks to hit Kabul as the majority of foreign combat troops withdraw from Afghanistan by the end of the year.
In recent weeks, insurgents have launched attacks on military convoys and compounds housing foreign service companies and their international employees.
British embassies and staff have been targeted in trouble-spots around the world in recent years, with Kabul among them.
In August 2008 a car bomb attack near the embassy in the Afghan capital left four people dead, although none were British personnel.
In November 2011 the embassy in the Iranian capital Tehran was closed after it was stormed by a mob of protesters.
In April 2010 the then British ambassador to Yemen, Tim Torlet, escaped unharmed when a suicide bomber wearing a school uniform detonated an explosives belt as he made his way to work in the capital Sana'a. The following October, his deputy, Fionna Gibb, escaped a rocket attack in the city.
Last year the embassy in the Arab country was closed by the Foreign Office during the Islamic festival Eid, amid ''continuing security concerns''.
On November 2003 the British Consul-General in Turkey, Roger Short, was killed by a suicide car bomb in Istanbul blamed on al Qaida which left 60 others dead and hundreds injured.
In a regular statement updating the House of Commons on the latest situation in Afghanistan, Defence Secretary Michael Fallon said today's killings were "a tragic reminder that there are some who still seek to undermine the progress that has been made towards peace and security in that country".
The two Britons were members of a G4S security team.
The firm said: " At 10.20am local time this morning in Kabul a suicide vehicle-borne improvised explosive device (IED) was detonated targeting a British Embassy vehicle moving through the city.
"A local member of the embassy staff and an expatriate G4S security team member were both killed in the explosion. A second British member of the G4S security team was injured.
"An investigation into the incident is under way and therefore we are unable to comment further at this time.
"Next of kin have been informed and we will continue to provide them with support. Our thoughts and most heartfelt condolences are with the families, friends and colleagues of those involved in this tragic incident."