British officials admitted last night that they had no way of verifying claims that one of five Britons held by militants in Iraq had killed himself. Officials have lost contact with the hostage-takers.

A tape, purporting to come from the kidnappers, alleged that a man known as Jason died on May 25. Efforts to corroborate the claim were continuing last night but a senior official told The Times that the security forces had no reliable way of communicating with the group. A previous intermediary had provided only intermittent and inadequate information, the official said.

Gordon Brown said last night that the “abhorrent film” would add to the anguish of the hostages’ families. He said that he had discussed the kidnapping with his Iraqi counterpart, Nouri al-Maliki, in Baghdad, adding: “I call on the hostage-takers to release these people who have been held in captivity immediately . . . These men have suffered enough.”

It is understood that a lack of reliable information had led to the abandonment of two rescue attempts. The five were kidnapped in May last year by militants disguised as police officers. Another hostage - understood to be a father-of-two from the Glasgow area called Alan - was shown in moving footage pleading for his release. “Physically, I’m not doing well. Psychologically, I’m doing a lot worse. I want to see my family again,” he said. “I would like for the British Government to please hurry.”

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