A seven-year-old girl is fighting for her life and four women and another child were injured during the memorial service at St Aloysius Church in Euston on Saturday.
Sir Keir said the attack and other shootings, such as the killing of five people in Keyham, Plymouth in 2021, show the need for reforms to gun laws.
The former Director of Public Prosecutions told LBC: “We’ve had these incidents from time to time with guns, there was one down in Plymouth last year, and every time there is , there’s evidence, which I am concerned about, that people have access to guns that they shouldn’t have access to.
“Now with illegal access that’s a pure matter of criminal law. But other people where better checks should be taken on circulation of guns. So I think that we need to look again as to whether those laws are strong enough.”
Sir Keir said that he had been in “constant contact with police” and involved in intelligence briefings over the shooting in Euston.
He added: “There are many illegally-owned guns out there and there are legally-owned guns, which I don’t think should be in the hands of the people who are legally owning them.”
Sir Keir said he does not back an outright ban on gun ownership, saying that exemptions should be allowed for famers and sports shooting clubs.
A man has been arrested on suspicion of attempted murder in connection with the Euston shooting.
Police on Sunday said they were seeking information on a black Toyota.