A suicide car bomber hit a European Union police training mission convoy near Kabul's international airport on Sunday, killing three people.
The Taliban has claimed responsibility for the attack that took the lives of a British security contractor and two women bystanders.
The Afghan Interior ministry says 18 people including civilians and three foreigners, working for the EU police mission, were also injured in the blast.
Britain's Foreign Secretary Phil Hammond called the bombing cowardly. He added that such attacks "must not stop the people of Afghanistan, with the support of the international community, working towards a more peaceful future."
An eyewitness said he was inside his shop when the explosives-laden Toyota Corolla drove into the street and detonated.
The senior doctor at a hospital said he had received 25 civilian victims.
Taliban militants have carried out a series of attacks in Afghanistan since most foreign troops pulled out of the country last year.
The attack in Kabul follows a Taliban assault on a guesthouse in the capital a day earlier that killed 14 people, including nine foreigners.
Also on Sunday, the Taliban said it released dozens of hostages captured Saturday in Paktia province. Two people remain in militant custody amid accusations they have ties to the Afghan government.
Ayaz Gul in Islamabad contributed to this report.