Graham Hall, who has exposed drug-dealers and badger-baiters in earlier films, faces plastic surgery after the horrific attack scarred a large area of his back. Channel 4, which screened Mr Hall's programme as part of its Dispatches series last year, confirmed he had been branded and offered him "every support".
Mr Hall, 43, filmed ALF members boasting about making bombs and choosing sites for violent attacks. He said last night he had been kidnapped on 26 October, after arranging to meet a man who said he could expose a dog- fighting ring.
Mr Hall told the Mail on Sunday that as soon as he had met the contact he was bundled into a van, a hood was put over his head and what seemed to be a gun was pressed against him. "I thought I was being taken to my execution," he said.
Among the voices of the attackers, Mr Hall recognised one connected with previous ALF activity that he had witnessed. The gang took him to a house where he was tied to a chair for some hours.
"The next thing I knew someone had pushed my head down and I felt an excruciating pain across my back," he said. He had been branded by a massive iron spelling the letters ALF.
Hall said that after nearly 12 hours he was thrown out on to the road, with a final warning that if he told the police or the media "you'll die".
A Channel 4 spokesman said last night: "Graham Hall is one of the country's best undercover journalists. His undercover work in last year's award- winning Dispatches programme "Inside the ALF" was central in exposing the dangerous fanaticism at the core of the organisation." A spokesman for West Mercia Police said it would look into the reports.