The High Court in Belfast today turned down an application to have a Northern Bank staff member released by police who have been questioning him about the £26.5 million robbery last December.
Chris Ward (24) from Poleglass, West Belfast, has been in custody since his arrest a week ago.
Mr Justice Hart upheld a ruling last night by a lower court in Northern Ireland that Mr Ward could be questioned for a further 48 hours - the first time in Northern Ireland that a suspect has been held by police for questioning for more than seven days.
Mr Ward was held hostage by the gang which carried out the robbery on the bank's headquarters in the centre of Belfast just before Christmas.
Police announced this afternoon that another man had been arrested for questioning about the bank raid. The 35-year-old man was detained in Belfast. Detectives investigating the robbery have already charged three men, one of them for carrying out the raid.
Today's court challenge was mounted on the grounds that Mr Ward and his solicitor were excluded from part of the hearing before a Crown Court judge last night.
Following last night's court hearing, Mr Justice Hart granted an interim order at 1.15am today preventing police questioning Mr Ward further until the matter had been resolved before him this morning.
At today's hearing, Frank Donoghue QC, for Mr Ward, argued that the suspect and his solicitor should not have been excluded by the Crown Court judge while police explained why they wanted to hold him further.
Mr Ward and his solicitor were asked to leave while a senior detective detailed five areas of questioning they wished to put to Mr Ward if they were granted a further 48 hours to question him.
The court had been told they had already questioned him about 14 topics in relation to the robbery.
The QC said that in doing so, the judge had exceeded the remit of the Terrorism Act 2000.
Almost none of the £26.5 million taken in the robbery, blamed by PSNI Chief Constable Sir Hugh Orde on the IRA, has been found.