... Somalia's version of the Taliban, functioning as an arm of al-Qaeda...
The 45-year-old was held at the airport in the capital, Mogadishu, yesterday after travelling from the UK via Nairobi in Kenya, according to reports.
He is alleged to have told immigration officials he was planning to go to Kismayo in southernSomalia, a port city held by al-Shabaab, which is affiliated to al Qaeda.
The Foreign and Commonwealth Office (FCO) said it was aware of reports about the Briton's arrest.
Britons are thought to make up about a quarter of the estimated 200 foreigners fighting with al-Shabaab inSomalia, according to a recent Royal United Services Institute report.
The FCO advises against all travel to war-tornSomalia.