Two British adults and a child of another British national were killed in the Kabul airport bombing, the UK said on Friday.
Two other Britons were injured in the atrocity, Foreign Secretary Dominic Raab said.
He said they were “innocent people” caught in the tragedy carried out by “cowardly terrorists”.
“I was deeply saddened to learn that two British nationals and the child of another British national were killed by yesterday’s terror attack, with two more injured,” Mr Raab said.
“These were innocent people and it is a tragedy that as they sought to bring their loved ones to safety in the UK they were murdered by cowardly terrorists.”
The announcement was the first confirmation that British nationals were killed.
At least 90 Afghans and 13 US troops were killed, and more than 150 Afghans injured, US officials said.
The death toll was expected to keep rising, with some Afghan officials saying it could reach 115.
ISIS-K, the local affiliate of militant group ISIS, has claimed responsibility for the suicide bombing.
“Yesterday’s despicable attack underlines the dangers facing those in Afghanistan and reinforces why we are doing all we can to get people out.
"We are offering consular support to their families,” Mr Raab said.
“We will not turn our backs on those who look to us in their hour of need, and we will never be cowed by terrorists.”
Evacuation flights from Afghanistan began again on Friday.
However, concerns over further attacks led a number of countries, including Canada, Denmark, Germany, the UK and Sweden, to announce they are cutting short their evacuation schedules.