Paul Bigley said on Saturday armed intelligence officers had raided his home, copied data from his computer and forced him to make a five-page statement about his activities.
While he said the raid happened two days ago, a spokeswoman for the British Foreign Office said neither British nor Dutch officials had carried out such a raid.
Paul said the raid had made him feel like a criminal and had wasted time in the race to save his brother's life.
"I have lost 48 hours of my quest to get Ken free because of all this nonsense," he said. "It was incredible."
London's Independent daily also reported the raid, and said the intelligence officers interrogated Paul about links with the group holding his brother captive.
Paul signed a five-page statement and the officers sent information from his computer hard disc to London for analysis, the newspaper said.
Paul has been a vocal critic of UK Government efforts to secure his brother's release.
One of his neighbour's said Paul felt intimidated by the raid. "He feels as though he is being treated like a criminal," the neighbour said.
Kenneth Bigley was captured more than a fortnight ago with two Americans, both of whom have since been killed by the captors.
The 62-year-old engineer has made two videotaped appeals, in which he begged UK Prime Minister Tony Blair to meet the demands of the captors and save his life.