A 20-year-old man was arrested at 6am in Cardiff on Monday (September 25) and police are searching a property in the city, while the man is held in custody at a south London police station.
Counter terror police have made seven arrests in connection with the September 15 incident, with three of the men since being released with no further action.
Ahmed Hassan, aged 18, appeared at Westminster Magistrates Court on Friday (September 22), charged with attempted murder and causing an explosion likely to endanger life or cause serious injury to property.
The Sunbury resident will appear at the Old Bailey on October 13.
A 25-year-old man and a 30-year-old man are also being held in police custody under the Terrorism Act in connection with the District line bombing.
A 17-year-old boy was released with no further action of Friday (September 22) while two men aged 48 and 21 were released by police on Thursday (September 21).
Police are continuing to search an address in Sunbury in addition to the Cardiff address.
Searches at properties in Hounslow, Newport and Stanwell have been completed, according to Metropolitan Police.
Keep up to date with the latest news in west London via the free getwestlondon app.