The Briton, who has not been identified, works for a foreign company building a highway from the southern city of Kandahar to the western city of Herat, said local police chief Allah Uddin Noorzi.
The engineer was kidnapped yesterday after an attack on the convoy in which he was travelling. A Filipino employee of the company escaped and was later found hiding under a bridge.
Noorzi said four policemen providing security for the convoy were killed, although Interior Ministry spokesman Latfullah Mashal later said that only three officers had died.
The situation remained unclear in many ways. The police chief blamed the Taliban for the kidnapping, but Mashal said it was the work of a criminal gang.
Mashal said no contact had been made with the kidnappers and that a delegation from Kabul was rushing to the area.
A spokesman for the British Embassy in Kabul said there had been "an incident involving a British citizen" in Afghanistan, but declined to give more details.
"We are urgently seeking more information from the government of Afghanistan," he said.
Major Andrew Elmes, a spokesman for a Nato-led peacekeeping force in the region, confirmed that a Briton had been abducted.
"We are now standing by ready to give any assistance," he said.
He said Nato troops had set up checkpoints in the area and were searching for signs of the abducted pair.
The kidnappings comes just weeks after a Lebanese engineer building another road in southern Afghanistan was abducted. He was later released unhurt.
Militants have stepped up attacks in the lead-up to landmark legislative elections on September 18. More than 1,100 people have been killed in the past six months.Reuse content