The trial over an alleged plot by a terror cell to bomb Britain was suddenly halted today when a key defendant refused to continue giving evidence, claiming that security forces had visited his family in Pakistan.

Omar Khyam, 24, who is accused with six others of planning a bombing campaign of Britain with more than half a tonne of fertiliser explosive, said that he was now worried about the safety of his family, the Old Bailey heard.

Last week the jury was told by Mr Khyam how he had gone to Pakistan to receive military training.

During his two days in the witness box Mr Khyam said: "The ISI (Pakistan's Intelligence service) was setting up training camps in what we called Free Kashmir, funding it (them) with money and weapons, and people that would train people, and logistical supplies, everything. It was because Pakistan didn’t want their soldiers dying."

The jury were due to hear today about the fertiliser that was discovered in a West London storage depot. Mr Khyam had denied planning operations for English targets, but had yet to explain why he allegedly bought it.

Asked by his counsel, Joel Bennathan, whether he had bought the fertiliser with the help of others, he said he would not go on.

Mr Khyam, who was wearing a yellow open-neck shirt, said: "Before we go on to that topic, I just want to say the ISI in Pakistan has had words with my family relating to what I have been saying about them.

"I think they are worried I might reveal more about them, so right now, as much as I want to clarify matters, the priority for me has to be the safety of my family so I am going to stop.

"I am not going to discuss anything related to the ISI any more or the evidence."

The court was adjourned for lawyers to consider the situation.

After more than an hour, the court resumed for the judge to warn Mr Khyam that the jury could draw inferences from his refusal to continue.

Sir Michael Astill told the defendant: "If you refuse to answer questions, the jury may draw such inferences as appears proper from your failure to do so."

Mr Khyam answered "yes" when asked if he understood.

The jury was then told to leave the courtroom. The case was later adjourned until tomorrow.

Mr Khyam, his brother Shujah Mahmood, 19, Waheed Mahmood, 34, and Jawad Akbar, 23, all from Crawley, Sussex, Salahuddin Amin, 31, from Luton, Bedfordshire, Anthony Garcia, 24, of Ilford, east London, and Nabeel Hussain, 21, of Horley, Surrey, deny conspiring to cause explosions likely to endanger life between January 1, 2003 and March 31, 2004.

Mr Khyam, Mr Garcia and Mr Hussain also deny a charge under the Terrorism Act of possessing 1,300lb (600kg) of ammonium nitrate fertiliser for terrorism.

Mr Khyam and Shujah Mahmood further deny possessing aluminium powder for terrorism.

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