The attacker, Usman Khan, was apparently attending the event and had returned for the afternoon session when he started stabbing people.
But Labour leader Jeremy Corbyn will blame budget cuts for "missed chances to intervene" in a speech on Sunday.
This undated file handout photo obtained from West Midlands Police on February 1, 2012 shows Islamist Usman Khan, then 20, who was jailed on February 9, 2012 with others after admitting to being involved with a group of fundamentalists who plotted a spate of mail bomb attacks during the run-up to Christmas in 2010.
Khan, a British national, had been handed an indeterminate sentence for the protection of the public in 2012 of at least eight years in prison.
This sentence would have allowed him to be kept in prison beyond the minimum term.
In 2013, Khan and three other men argued that they shouldn't have received indeterminate sentences for public protection (IPPs) - special sentences meant to keep prisoners beyond their original minimum term.
"To the best of my knowledge, he was complying with those conditions", he added.
Khan, 28, was on licence and wearing an electronic monitoring tag when he attended a conference on prisoner rehabilitation hosted by Cambridge University scheme Learning Together at Fishmongers' Hall near London Bridge.
He appeared as a "case study" in a report by the initiative.
"I visited one of his prison projects and his deep commitment to prisoner education and rehabilitation was deeply impressive".
Asked why this has not happened under the last years of Conservative government, he added: "I'm a new Prime Minister, we take a different approach".
Merritt worked as a co-ordinator for the group.
The other woman who was killed has not yet been identified.
Two people remain in hospital, and one has been discharged.
Mr Basu said officers had been working "flat out" to try to establish the "full circumstances" of the stabbing.
In another post subsequently deleted, he said his son "would not wish his death to be used as the pretext for more draconian sentences or for detaining people unnecessarily".
"It's already clear that this cowardly act was immediately countered by some incredible acts of bravery both by members of the public and by police officers", said the assistant police commissioner.
The Ministry of Justice says it will now urgently review the policy for releasing convicted terrorists. Judges could still hand down life sentences or IPPs for unsafe offenders.
A former Labour minister Yvette Cooper said on Twitter that the government was "warned about the risks" of scrapping a program introduced by Labour that handed unsafe prisoners an indefinite sentence.
Despite the law change coming into force before Khan's appeal, he could only be sentenced under legislation in force when he committed his offences.
Mr. Johnson blamed Labour for the 2008 law which was backed at the time by Mr. Corbyn.
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