Sir Keir Starmer demanded a crackdown on the online sale of dangerous weapons as he pledged to "double down" on efforts to tackle knife crime.
"Far too many knives are too easily available, whether that's online, whether that's through the post," he said.
The Prime Minister highlighted examples of knives "bought online being posted through without any markings, and then people picking them up without any identification, and then those knives being used".
It is "just too shocking - that needs to be dealt with," he stressed.
At a summit at No10 on Monday, attended by Luther star and knife crime campaigner Idris Elba, Sir Keir said: "Before the election, I made commitments to people around this table and others in relation to a determination to deal with knife crime. And it is a determination."
He added: "Now I want to double down on that commitment. It wasn't a commitment said before the election that's then forgotten the other side of the election - I think many people are far too familiar with that kind of politics."
The Prime Minister welcomed Elba to Downing Street on Monday to launch a coalition to drive forward initiatives to keep dangerous weapons off British streets and stop young people from being dragged into violent gangs.
The Hollywood actor told the knife crime meeting at No 10: "Talk is good, but action is important."
He said it was important there was a coalition of interests involved in the summit.
"We needed joined-up thinking, we needed so many different perspectives: parents' perspectives, youth workers' perspectives, charity organisations, governance, policing.
"We need all of these perspectives to sit around the table when you think about this.
"We aren't going to end knife crime. We can't, that's not realistic.
"But we can tackle the attributes towards it. At the centre of it is obviously young people - my son's 10, and I'm hoping the work that we do annually, keep pushing, (can) help him by the time he's 16.
"There are kids right now that are 16-24, they're in that cycle right now that we might not be able to help, but with our joined-up thinking we can help future generations.”
Pooja Kanda, whose son Ronan was murdered in 2022 with a weapon bought online, told the No 10 summit she was "horrified, absolutely horrified" by what was available to her son's killer.
"It was as easy as picking up a loaf of bread from a supermarket," she said.
Calling for action, she said: "I'm not a policy expert in reducing crime. I'm a grieving mother.
"But I will do all I can to ensure that the failings that led to my son's murder are corrected."
She added: "It blows my mind at how easy it is to purchase these weapons and have them delivered to your door with absolutely no checks or accountability held."
Sir Keir suggested ministers will look at the punishments for those caught with knives.
The Prime Minister told the knife crime summit in Downing Street: "We also need to deal with the sanctions for those that are found with knives."
But there is also a need to "try to reach into the lives of young people who may be going off the tracks and trying to bring them back before they do so".
He acknowledged "there is no quick fix" to the problems and urged the campaigners and experts around the table in No 10 to be "brutally frank" with their views.
Actor @idriselba has joined the prime minister @Keir_Starmer to launch a new anti-knife crime coalition in Downing Street.
— Sky News (@SkyNews) September 9, 2024
It is currently in the process of banning ninja swords and strengthening the law on online knife sales. https://t.co/PAiZ4D1jU3
📺 Sky 501 pic.twitter.com/s7qzxa5gae
The coalition will bring together campaign groups, families of people who have lost their lives to knife crime, and young people who have been affected by it - as well as knife crime campaigner Elba and home secretary Yvette Cooper.
Technology companies, sporting organisations, the health service, and the police are also involved in what is expected to the first annual knife crime summit.
The coalition will work with experts to develop an understanding of what causes young people to be dragged into knife crime.
Decisions will be especially relevant in London with Office for National Statistics data showing that 14,626 knife offences were recorded by police in the capital over the 12 months to the end of last December.
Knife and gun crime in London both leapt by 20 per cent last year amid a surge in blade robberies, teenage homicides and firearms offending.
Ministers have already taken steps to ban so-called ninja swords, and plan to strengthen the laws around the online sales of knives.
Sir Keir previously met with Elba and the King to put forward Labour’s strategy to reduce knife crime in the capital.
Commander Stephen Clayman, the national policing lead for knife crime, has been tasked with leading a rapid review to understand how these weapons are sold online and delivered to under 18s, and to close loopholes in the law.
He will report back to the home secretary by the end of the year.
Home Secretary Yvette Cooper said: "Getting weapons off our streets and making sure there are tough and clear consequences for violence are vital.
"And we also need to prevent young people heading down this path - that means offering young people more hope, more opportunities."
The announcement is the first step in the Government's 10-year plan to tackle knife crime, which will be central to its mission to keep our streets safe.
