Months of research across the country show how areas the Taliban threaten or control have surged since foreign combat troops left in 2014.
The Afghan government played down the report, saying it controls most areas.
But recent attacks claimed by Taliban and Islamic State militants have killed scores in Kabul and elsewhere.
Afghan officials and US President Donald Trump responded by ruling out any talks with the Taliban. Last year Mr Trump announced the US military would stay in the country indefinitely.
How was the research carried out?
The BBC investigation – conducted during late 2017 – provides a rare snapshot of the security situation in every Afghan district between 23 August and 21 November.
A network of BBC reporters across Afghanistan spoke to more than 1,200 individual local sources, in every one of the country’s 399 districts, to build up a comprehensive picture of all militant attacks over that period.
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