Ex-Metropolitan Police commissioner Ian Blair has died at the age of 72.
Christ Church, Oxford, where Lord Blair studied English and later became an honorary student, confirmed his death on Friday.
The crossbench peer, who led the force during the July 7 bombings, served as Britain's most senior police officer from 2005 to 2008, overseeing Scotland Yard during one of its most turbulent periods.
His tenure was marked by the 2005 London Islamist terror attacks that killed 52 people and the fatal shooting of Jean Charles de Menezes at Stockwell underground station, when anti-terrorist officers mistook the Brazilian electrician for a suicide bomber.
Former Metropolitan Police Commissioner Lord Ian Blair has died
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Born in Chester in 1953, Lord Blair joined the Metropolitan Police in 1974 as part of its graduate entry scheme.
He rose through the ranks to become chief constable of Surrey Police before returning to the Met as deputy commissioner in 2000, taking the top job five years later.
Current Metropolitan Police Commissioner Sir Mark Rowley paid tribute, saying: "Lord Ian's passing is a huge loss to the police family. He dedicated his life to policing and will be remembered as one of the most influential police leaders of recent decades."
Rowley added that Lord Blair "oversaw some of the most challenging moments in our history not least the response to the horrific terror attacks on July 7 2005 which we commemorated earlier this week."
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Ian Blair with Gill Hicks, a survivor of the July 7 London bombings
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London Mayor Sir Sadiq Khan said: "Lord Ian Blair dedicated over 30 years to public service as a police officer rising through the ranks from a graduate recruit to becoming Commissioner of the Met. My thoughts are with his loved ones."
Lord Blair's leadership came under intense scrutiny following the shooting of Jean Charles de Menezes on July 22 2005, two weeks after the 7/7 attacks.
The innocent Brazilian was killed by anti-terrorist police who mistook him for a suicide bomber, exposing what one observer described as "an extraordinary degree of operational disarray."
Critics said Lord Blair "did not know what was going on, seemed unable to find out, and had nobody who was prepared to tell him" after he finished work that night without understanding the full extent of the catastrophe.
Former Prime minister Gordon Brown with Met Commissioner Sir Ian Blair
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Despite calls for his resignation, he remained in post but faced mounting criticism for allegedly hindering the Independent Police Complaints Commission investigation and not being informed of Mr de Menezes' innocence for 24 hours.
His tenure ended abruptly in 2008, when Conservative Mayor Boris Johnson effectively dismissed him, telling him privately he had no confidence in his work.
The unprecedented move exposed deep tensions between City Hall and Scotland Yard, with Lord Blair becoming the first Met commissioner to be forced from office in such circumstances.
He was made Lord Blair of Boughton in 2010, taking a seat as a crossbench peer, having been knighted in 1999.
Metropolitan Police Commissioner Sir Ian Blair during a meeting of the Metropolitan Police Authority in 2008
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Known for his liberal outlook and efforts to modernise the force, he was dubbed "the PC Pc" by critics who accused him of politicising policing and prioritising image over operational effectiveness.
Rowley noted that Lord Blair "continued to be a friend of policing and police officers" after leaving the force.