A 30-year-old man was arrested in the central city of Birmingham on suspicion of preparation of terrorist acts, London's Metropolitan Police said in a statement.
A dozen people have been arrested since 52-year-old Briton Khalid Masood rammed his car into pedestrians and tried to force his way into the parliament on Wednesday, killing four people and leaving 50 injured before he was shot by police.
Nine people have been released without charge, while a 58-year-old man remains in custody and a 32-year-old woman has been released on bail.
Police on Saturday admitted they may never know why Khalid Masood chose to drive his hire car into pedestrians before crashing into the fence of the Houses of Parliament and fatally stabbing a policeman.
"We must all accept that there is a possibility we will never understand why he did this. That understanding may have died with him," said senior counter-terrorism officer Neil Basu.
Police believe he acted alone on the day, although investigators are trying to find out whether he was encouraged or directed by others.
The terror ISIS group claimed that one of its "soldiers" carried out the attack, the worst in Britain since the July 2005 bombings which left 52 people dead.
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