The Metropolitan Police said they had reviewed security ahead of the vote, putting in place a "specialist and highly flexible operation" which can be deployed as needed.
One further man, arrested on suspicion of drugs and firearms offences, was bailed, the Metropolitan Police said.
His house had been raided this year by police as part of a disruption tactic used during active counter-terrorism investigations.
The Italian newspaper Corriere della Sera said in a report that the Italian intelligence services reported Youssef's movements to their British counterparts.
London authorities announced on Wednesday that three more men had been detained in connection with last weekend's deadly terrorist attack in the city.
The police say Butt was one of the three men, who rammed a van into pedestrians on the London Bridge before launching a stabbing spree in bars and restaurants at nearby Borough Market on Saturday night.
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Apple also introduced to developers a new AR kit that it believes will make the company a leader in the augmented reality domain. All in all, it was a big day for Apple , with more big news expected tomorrow, on Day 2 of the worldwide developers conference .
The van-and-knife attack also injured nearly 50 people who were enjoying their Saturday night in a crowded part of London known for its bars and restaurants.
Thomas was walking with his girlfriend Christine Delcros across the bridge on Saturday night while visiting London on holiday.
O'Leary, who had a child with Redouane, said she was "deeply shocked, saddened and numbed" by what he had done before being shot dead by authorities in Borough Market, along with Butt and their accomplice, Youssef Zaghba.
Three men suspected to be involved in terror activities were arrested in London on Wednesday.
A 22-year-old Moroccan-Italian, Zaghba was stopped at Bologna Airport in March 2016, heading to Turkey on a one-way ticket. However, they did not arrest him because he held an Italian passport, and "having extremist material on a phone isn't against the law in Italy", according to the CNN report.
Fahad Khan, 36, a cousin of Butt's wife Zahrah Rehman, claimed his younger relative watched the hateful tirades of preachers online and supported the Islamic State terror group. "But there was no intelligence to suggest that this attack was being planned and the investigation had been prioritised accordingly".
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