Two of the men shot dead by police following the attack on London Bridge and Borough Market on Saturday are seen in this undated combination image of two photographs, received in London via the Metropolitan Police in London on June 5, 2017. [Photo/Agencies]

The news raises questions about the police's judgment and increases pressure on Prime Minister Theresa May, who three days before a national election is facing criticism for overseeing cuts to police during her years as interior minister.

In Britain's third Islamist attack in as many months, three men on Saturday rammed a van into pedestrians on London Bridge before running into the Borough Market area, where they slit throats and stabbed people indiscriminately. Seven people were killed and dozens wounded.

All three attackers were shot dead by police, who made at least a dozen arrests in east London on Sunday and carried out further raids on Monday.

Police on Monday named two of the attackers and said they were trying to identify the third. One, 27-year-old Khuram Shazad Butt, was a British citizen born in Pakistan who had already been investigated by police and Britain's domestic spy agency MI5.

"However, there was no intelligence to suggest that this attack was being planned, and the investigation had been prioritised accordingly," police said.

Another attacker, 30-year-old Rachid Redouane, went by the alias Rachid Elkhdar and claimed to be Moroccan or Libyan, police said. He and Butt lived in the same area of east London.