The jury in the trial of seven men accused of plotting to bomb UK targets was sent home after a defendant refused to give further evidence in court.
Omar Khyam said Pakistani security officials had contacted his family
Omar Khyam, 24, of Crawley, Sussex, said he feared for the safety of his family in Pakistan after they were contacted by security services there.
The judge told Mr Khyam the jury could "draw inferences" from his refusal.
He and six others deny conspiring with a Canadian to cause explosions. The trial was adjourned until Tuesday.
Mr Khyam was the first of seven Britons who deny conspiring to cause explosions to give evidence at the Old Bailey.
Two days' evidence
He had entered the witness box last week and spent two days talking about his time in Pakistan, where he received military training, and then "working for the cause" to free Islamic lands.
After speaking on Friday about raising money for Afghanistan using fraud, he was due to talk about fertiliser linked to the alleged plot on Monday morning.
But after being asked by his counsel, Joel Bennathan, whether he had bought the fertiliser with the help of others, Mr Khyam said he would not continue.
"Before we go on to that topic, I just want to say the ISI [Pakistani secret services] in Pakistan has had words with my family relating to what I have been saying about them," he told the court.
"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 to consider the situation, after which the judge, Sir Michael Astill, warned Mr Khyam the jury could "draw inferences" from his refusal to continue.
"If you refuse to answer questions, the jury may draw such inferences as appears proper from your failure to do so," he said.
The defendant answered "yes" when asked if he understood.
The jury was then told to leave the courtroom.
BBC correspondent Andy Tighe said there was a question mark over whether Mr Khyam would be forced to be submitted to cross-examination.
The judge would make his intentions clear when the jury reconvened at 1030 BST on Tuesday, our correspondent said.
"The options are that Omar Khyam could maintain his position, not answer any further questions and not go into the witness box - no-one can be forced to give evidence if they don't want to.
"But the next step in this process, after you've been questioned by your own barrister, is that cross-examination can begin so that could be the next step.
"The question is, will he be submitted to cross-examination by the prosecution barrister tomorrow or not?"
The seven defendants were arrested in March 2004 when 600kg of fertiliser was found stored in a west London depot.
It is alleged they discussed bombing targets including pubs and nightclubs.
It is alleged the men plotted between 1 January 2001 and 31 March 2004 to set off a series of bombs.
Mr Khyam, his younger brother Shujah-Ud-Din Mahmood, 18, and Waheed Mahmood, 33, from Crawley, are accused of conspiring to cause an explosion likely to endanger life contrary to section 3 (1)(a) of the Explosive Substances Act 1883.
Jawad Akbar, 22, from Crawley and Uxbridge, Anthony Garcia, 27, from Ilford, east London, Nabeel Hussain, 20, from Horley, Surrey, and Salahuddin Amin, 30, from Luton, Bedfordshire, face the same charge.
Mr Khyam, who has also lived in Slough, Mr Garcia and Mr Hussain are also charged under the Terrorism Act 2000 of possessing an article for terrorism - namely 600kg of ammonium nitrate fertiliser between 5 November, 2003 and 31 March, 2004.
Brothers Mr Khyam and Mr Mahmood also deny having aluminium powder, which is an ingredient in explosives, between the same dates.