A man has been arrested on suspicion of terror offences by detectives investigating the London Bridge attack following an early morning raid at an address in east London.
The 30-year-old was held on suspicion of the commission, preparation or instigation of terrorist acts during a search of a property in Ilford and taken for questioning at a south London police station, Scotland Yard said.
The arrest comes as security services face growing questions over their monitoring of the three attackers in the lead up to the attack.
A Metropolitan Police spokesman said: "Detectives investigating the London Bridge terror attack have carried out a search warrant at an address in east London in the early hours of Wednesday June 7.
"Officers from the Met's Counter Terrorism Command, supported by officers from the Territorial Support Group, entered the address in Ilford at around 1.30am.
"A 30-year-old man was arrested on suspicion of the commission, preparation or instigation of terrorist acts (contrary to section 5 Terrorism Act 2006) and has been taken into custody at a south London police station under the Terrorism Act 2000.
"A search of the address is ongoing."
On Tuesday the third attacker was named as Youssef Zaghba, an Italian national of Moroccan descent, who was living in east London.
Zaghba, Pakistan-born British citizen Khuram Shazad Butt, 27, and Rachid Redouane, 30, who claimed to be Moroccan-Libyan, launched a murderous rampage around London Bridge and Borough Market on Saturday night.
According to the mother of 22-year-old Zaghba, he became radicalised online, echoing concerns raised by Prime Minister Theresa May that the internet can be fertile ground for breeding extremism.
"We have always been checking his friendships and verifying that he was not trusting the wrong people, but he had the internet and from there he got everything," Valeria Khadija Collina told L'Espresso in Bologna.
Zaghba was stopped at Bologna's airport trying to fly to Turkey in March last year over concerns he was intending to travel on to Syria, according to reports.
The youngest of Saturday's attackers is said to have told Italian authorities "I'm going to be a terrorist", while officers reportedly found Islamic State-related material on his mobile phone when they intercepted him.
Counter-terror agencies are already facing intense scrutiny after it was revealed Butt had been known to security services.