Grant Bailey is variously described as a security consultant and a worker for an unidentified organisation that is not government related.
Mr Bailey is believed to have returned to Afghanistan in September, after the Taliban takeover in August.
He is in his 50s, believed to be from the south of England and has spent several years in Afghanistan.
“We are aware of the detention of a British national in Afghanistan and have been touch with their family to support them,” a Foreign, Commonwealth and Development Office spokesman said.
The spokesman added that assistance is remote as all embassy staff left in August as part of a wider operation to get all Britons, international citizens and Afghan allies out of the country.
A nation on the brink
Millions of Afghans face starvation and up to one million children are at risk of starvation, a group of international aid agencies have warned.
They say that up to 25 per cent of the country's 40 million population risk dying from famine.
Shortly after Taliban forces took control of Kabul, the UK said it would take up to 20,000 refugees overall and as many as 5,000 in the first year. More than 300 councils have pledged their support to families.
The UK government said on Thursday that its Afghan Citizens Resettlement Scheme programme would start in January, after months of waiting.
After the fall of Kabul, about 15,000 people, including 12,000 Afghans, were flown out of the country to the UK.
Most of the Afghans are in the UK under the Afghan Relocation Assistance Policy, a scheme for those who helped the British effort in the country.
Despite the UK government’s promise of a “warm welcome”, months later many of the new arrivals are still living in hotels, while they await permanent housing and residency documentation.