The journalist, who was abducted in Gaza on 12 March, spoke of how his captors would detonate the vest if authorities attempted to use force to try and win his freedom.
Upon seeing the video, the BBC immediately reissued its plea for his release, stating: "It is very distressing for Alan's family and colleagues to see him being threatened in this way.
"We ask those holding Alan to avoid him being harmed by releasing him immediately. We are keeping his family fully informed and offering them our continued support."
In the video, which was released by the kidnappers on the internet, Johnston says: "The situation now is very serious. As you can see I have been dressed in what is an explosive belt, which the kidnappers say will be detonated if there was any attempt to storm this area."
He added: "Captors tell me that very promising negotiations were ruined when the Hamas movement and the British Government decided to press for a military solution to this kidnapping."
Prior to the latest video being released, Hamas leader Ismail Haniya demanded that Johnston's captivity come to an end. He said: "We will not allow the continuation of the abduction of the British journalist. The issue of Alan Johnston must end."Johnston, from Scotland, was abducted 14 weeks ago by a group calling itself 'The Army of Islam'. He was the only Western reporter permanently based in Gaza.
According to the BBC, Johston's father, Graham said: "My family and I are obviously most concerned and distressed at this latest development. Our thoughts, of course, are with Alan in his present predicament. We earnestly request his abductors to release Alan, unharmed in any way."
