A 23-year-old man is being questioned over the 21 July attempted bomb attacks on London's transport network, after being arrested at Gatwick airport.
Shepherd's Bush station was one of the alleged targets on 21 July
He was detained after he had landed on a flight from Addis Ababa, Ethiopia.
The man, from north London, is being held in central London on suspicion of the commission, preparation or instigation of acts of terrorism.
Five men have been charged with conspiracy to murder following the failed attacks.
Police said they believed the suspect detained on Tuesday, who is from Tottenham, had been out of the UK since June.
He was arrested by anti-terrorism police, supported by Sussex officers, under the Terrorism Act 2000 at about 0500 GMT.
As well as the five charged with direct involvement in the alleged 21 July plot, 10 others face lesser charges including failing to disclose information about suspects and assisting them to evade arrest.
Dozens more people have been questioned under the Terrorism Act but released without charge.
The failed 21 July attacks took place two weeks to the day after four suicide bombers killed 52 passengers on the London transport network.