The 21-year-old had been arrested just before midnight on Saturday (September 16) in Kingsley Road, Hounslow.
Westminster Magistrates Court had granted a warrant for his detention until Thursday (September 21) when he was released with no further action.
Metropolitan Police continue to detain five suspects in the investigation into a bomb placed in a District line tube train, which detonated at Parson's Green station, injuring 22 people on Friday (September 15).
An 18-year-old was arrested at the Port of Dover by Kent Police at around 7.50am on Saturday (September 16) and Westminster Magistrates has granted police a warrant for his detention until September 23.
A 25-year-old man was arrested at an address in Newport, Wales on Tuesday (September 19).
On Wednesday (September 20) two more men from Newport, aged 30 and 48 were arrested at around 5.10am at a second address in the town.
A 17-year-old teenage boy was arrested on Thursday (September 21) in Thornton Heath, south London just after midnight.
All the men are currently being held in custody at police stations in London, while police continue to search an address in Surrey, one in Thornton Heath and two in Newport.
A search at another address in Surrey is now complete, according to Police.
Commander Dean Haydon, head of the Met Counter Terrorism Command, said: "This continues to be a fast-moving investigation. A significant amount of activity has taken place since the attack on Friday.
"We have five males in custody and searches are continuing at four addresses. Detectives are carrying out extensive inquiries to determine the full facts behind the attack.
"We anticipate that the searches will take some days to complete and may cause further disruption.
"However, it is important that we continue with these searches and I'd like to thank all those affected for their support, patience and cooperation.
"As police and our partners do all we can to confront the threat, public vigilance remains as vital as ever.
"We urge the public to report any suspicious activity to the police by calling us, in confidence, on 0800 789 321, or in an emergency by dialling 999.
"We are asking the public to look out for anything that seems out of place, unusual or just doesn't seem to fit in with day-to-day life.
"It may be nothing, but if you see or hear anything that could be terrorist-related, trust your instincts, then act, and call us."
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