A 30-year-old woman who was arrested at an address in Ealing on Thursday was freed by police at the weekend.
Officers continue to hold six people in custody at a south London police station, including a 22-year-old woman who was arrested in Hackney, east London.
She is being held on suspicion of the commission, preparation or instigation of acts of terrorism under the Terrorism Act 2000, a spokesman for the Metropolitan Police said.
The arrest, by officers from the Met’s Counter Terrorism Command, follows six others made on Thursday.
British Muslim convert Richard Dart, 29, is reportedly one of the six people arrested over the suspected terrorist plot.
Dart, who changed his name to Salahuddin al-Britani after converting, is believed to have been one of three people detained in Ealing.
He appeared in a BBC Three documentary, My Brother The Islamist, which described how Dart, originally from Weymouth, Dorset, had been converted by controversial cleric Anjem Choudary.
In the documentary he spoke of his support for jihad and sharia law.
Warrants of further detention were granted at Westminster Magistrates Court on Saturday.
The alleged plan involved Islamist extremists with potential targets in the UK, but was not linked to the Olympics.