In a statement on Thursday, he said: "It was an astonishing and deeply concerning decision that simply cannot go unchallenged. For victims, and all Londoners, it must be properly scrutinised in the courts. The chair of the Parole Board has welcomed this scrutiny.
"I will always put the safety of Londoners, and the needs of victims, first - which is why I instructed leading counsel to begin this process last week.
"Londoners need to know that those in authority are doing everything they can to keep them safe, and that means keeping dangerous individuals off our streets."