Eight of those detained can be held for seven more days.
Wednesday's court action represented the first time that police have used new anti-terrorism laws to hold suspects for more than 14 days without charge.
Police have until Thursday evening to interview the ninth, Umair Hussain, 24, from east London. Two suspects were released without charge.
An official with Britain's anti-terrorist unit confirmed that Tayib Rauf, who was videotaped at a grocery story in Birmingham just hours before he was arrested on August 10, was one of those released.
Mohammed Nazam, the owner of the grocery store where Tayib was taped and a friend of the Rauf family, also said that Tayib had been released.
Tayib's brother, Rashid Rauf, is being interrogated by law enforcement authorities in a town near the Pakistani capital, Islamabad, over his alleged role in the plot.
The second person released had initially been scheduled to have his detention hearing held early on Thursday. His identity was not disclosed.
Charged or released
The 11 were among 24 people arrested on August 10 in police raids in London, Birmingham and High Wycombe.
The others arrested have either been charged or released.
Timur Rustem, Umair Hussain's lawyer, said he had been ordered to be held until Thursday by a High Court judge.
Rustem said the judge dismissed a request that Hussain be held for a week.
Rustem said: "It is good to see the system works, where a High Court judge can in some cases grant the full seven days but at the same time use discretion where, for example, it's only a circumstantial case."