A 30-year-old man has been arrested in Birmingham by officers investigating the Westminster terror attack, the Metropolitan Police said.
A 58-year-old man is still in custody and a 32-year-old woman has been released on bail until late March after the attack that left four victims dead and 40 injured.
The attacker, Khalid Masood, was also shot dead.
Officers have carried out a total of 15 raids in east London, south London, Brighton, Surrey, Carmarthenshire, Birmingham and Manchester.
In total, twelve people have been arrested. Nine have been released with no further action.
“A 30-year-old man was arrested today, Sunday, 26 March at an address in Birmingham on suspicion of preparation of terrorist acts. He was detained under TACT (the Terrorism Act) and currently remains in police custody.”
It came as counter-terrorism police were searched a property in Birmingham where friends of Westminster terrorist Khalid Masood live.
Neighbours said unarmed officers first arrived at the three-storey maisonette at Bredon Croft in Hockley, Birmingham, on Saturday afternoon.
Officers were seen going in and out of the property which is just 150 yards from Quayside, where Masood lived until December last year.
A couple live at the house with their four young children, it was reported. Neighbours said Masood used to visit the property to see the couple and take their daughters, aged six and eight, to a mosque.
Masood killed two people, a Spanish teacher Aysha Frade and American tourist Kurt Cochran, as he drove a 4X4 down Westminster bridge before he fatally stabbed police officer PC Keith Palmer in the grounds of parliament and was fatally shot.