A third man has been arrested in connection with Friday’s Tube attack in Parsons Green.
The 25-year-old was arrested in Newport, Wales, Scotland Yard confirmed.
On Saturday an 18-year-old man was detained at Dover port and a 21-year-old believed to be Syrian Yahyah Farroukh was arrested in Hounslow.
A homemade bomb partially exploded in a train at Parsons Green station, injuring 30 people in rush hour.
The 25-year-old was arrested at 19:08 BST on Tuesday under section 41 of the Terrorism Act.
Police are now searching an address in Newport.
Metropolitan Police Commander Dean Haydon said: “This continues to be a fast-moving investigation.
“A significant amount of activity has taken place since the attack on Friday.
“We now have three men in custody and searches are continuing at four addresses.”
The Met’s Counter-Terrorism Command was supported by the Welsh Extremism and Counter-Terrorism Unit.