The prime minister was repeatedly challenged about how the Home Office, police and intelligence services dealt with the information relating to the attackers, after Boris Johnson, her foreign secretary, said MI5 had questions to answer.
Trudeau said Canada would stand with the U.K.in the fight against terrorism.
The country's official terror threat level had been set at "critical" in the days after the Manchester concert bombing on May 22 that killed 22 people - reflecting a judgment that an attack might be imminent because accomplices with similar bombs might be on the loose.
Butt was ticked off by parents for radicalising children in parks by paying them in candies to listen to his sermons. Rachid Redouane, 30 had claimed to be Moroccan and Libyan.
The main political parties suspended national campaigning after seven people were killed and nearly 50 hospitalized in a vehicle and knife attack Saturday night in the bustling London Bridge area.
The Islamic State (ISIS) terror group on Sunday claimed responsibility for the latest Islamic terror attack to strike Britain. "Geoff Ho is an absolutely first class reporter and a fine and decent man and our thoughts are with him and his family at this time", he said.
At the time of the attack he was working in a London restaurant and continued to have contacts with his mother in Italy, which he visited past year, Corriere wrote.
Butt was known to police and security services, and an investigation into him began in 2015, when one man called the terrorism hotline after concerns Butt had been been radicalised.
He was detained with only a small backpack, his passport and a one-way ticket to Istanbul. Police reportedly found Islamic State group propaganda on Zaghba's phone but did not have enough evidence to hold him.
Police named Kirsty Boden, 28, an Australian nurse who lived in London, as one of the victims on Tuesday.
In a statement, Chrissy's immediate family said she had worked in a homeless shelter until she had moved to Europe to be with her husband-to-be.
May tried to deflect the pressure onto Corbyn, a pacifist who has opposed some security legislation in parliament and expressed reservations in the past about police responding to armed attackers with "shoot-to-kill" tactics.
Several residents at the Elizabeth Fry flats in Barking, where a number of the arrests were made, said they recognised one of the suspected attackers from a photograph. "It is seeking to project its influence and maintain its relevance by inspiring, encouraging, and-though there does not appear to be evidence of this in the latest United Kingdom plots to date-directing attacks in Europe and elsewhere", he told AFP.
British Prime Minister Theresa May said Sunday that police had recently foiled five other plots.
Metropolitan Police Commissioner Cressida Dick told Sky News on Monday she would not release further details in what she described as a fast-moving investigation, not would she say whether authorities were familiar with the men ahead of the attack.
Early Monday, Trump tweeted: "Pathetic excuse by London Mayor Sadiq Khan who had to think fast on his "no reason to be alarmed" statement".