The 36-year-old was arrested on Tuesday by officers from the Metropoliton Police force's Anti-Terrorist Branch on suspicion of assisting an offender and alleged immigration offences, a spokesman said.
"The man has been taken to a central London police station where he remains in custody and will be interviewed," he said, adding that he was being held under the Police and Criminal Evidence Act.
The bungled incident mirrored the 7 July blasts, which killed 56 people, including four apparent human bombers, and wounded about 700.
Three subway trains and a bus were hit in co-ordinated morning rush hour blasts at four points across London.
Exactly two weeks later, a virtually identical attack was attempted with near-simultaneous bombings on trains at Shepherd's Bush station in west London, Oval station in the south, and near Warren Street station in the centre.
Like the 7 July attack, a bus was hit about an hour later, but this time in east London.
Also in custody is Hamdi Isaac,
who had fled to Italy
Hussain Osman, also known as Hamdi Issac, appeared in court on Friday charged with seven counts including conspiracy to murder and attempted murder in connection with the 21 July attacks after being extradited from Italy the day before.
His three alleged co-conspirators were all charged at the beginning of August.
Ibrahim Muktar Said, 27, Ramzi Muhammad, 23 and Yassin Omar, 24, were each charged with attempted murder, conspiracy to murder and making or possessing an explosive with intent to endanger life or cause serious injury.
Manfo Kwaku Asiedu, 32, was charged separately with conspiracy to murder and conspiracy to cause explosions likely to endanger life.
They are being held in jail ahead of their next scheduled court appearance at the Old Bailey criminal court in central London on 8 December.
The investigation into the two sets of attacks - one of the biggest police operations ever mounted in Britain - has not been able to establish any links between them.