Both suspects arrested in connection with the Parsons Green attack are being held under section 41 of the Terrorism Act 2000.
This law gives police the power to arrest someone suspected of terror-related offences without a warrant.
It also allows police to detain suspects without charge beyond the four days allowed for suspects connected to other crimes.
PA Graphics
The maximum period of pre-charge detention has varied but currently stands at a maximum of 14 days.
Detention must be reviewed at 12-hour intervals during the first 48 hours. After that, warrants for further detention must be obtained from a court.
Following the 7/7 bombings in 2005, the government attempted to get the maximum period of detention without charge extended to 90 days, but the proposal was defeated in the House of Commons.