Boris Johnson has pledged a new law to force venues that host large events to prepare for potential terror attacks.
The Prime Minister said public venues must look to "reduce their vulnerability" to terror attacks in the same way they would for a fire risk.
The move has been backed by the families of terror victims and comes after two people were killed in an attack at Fishermongers' Hall near London Bridge last Friday.
It would be a victory for the family of Martyn Hett - a victim of the Manchester Arena bombing - who have been campaigning for the change since the attack in 2017.
Announcing the proposals, Mr Johnson said there are steps that need to be taken make public spaces as secure as possible.
There has been a growing frequency of attacks on citizens in recent years, with the rise of so-called Islamic State in parts of Syria and Iraq encouraging lone-wolf and coordinated group offensives on the streets of Britain and other Western countries.
In May 2017, 23 people were killed when Salman Abedi, a radicalised Muslim, set off a homemade bomb as people were leaving an Ariana Grande concert at Manchester Arena.
Only two weeks later, three terrorists murdered eight after mowing people down in a van and then going on a stabbing spree through Borough Market in the London Bridge area.
Of the proposals, which he has pledged to enforce if the Conservatives win the General Election next week, Mr Johnson said: "The nature of threats faced by British citizens has changed in the evolution of modern extremism.
"It is no longer sufficient for public venues to prepare for accidental threats like fire. They need to reduce their vulnerability to people who seek to perpetrate violent acts too.
"We must not let the terrorists alter our way of life. In our open and tolerant society, the freedom for citizens to enjoy markets, concerts, gigs and restaurants must continue as before.
"But there are steps we can and will take to make public spaces as secure as possible. Already we are increasing the budget for counter-terrorism policing, including an extra £160 million a year, and we are developing a new counter-extremism strategy to reflect the changing face of extremism."http://players.brightcove.net/1348423965/default_default/index.html?videoId=6111097441001Boris Johnson, Jeremy Corbyn and Sadiq Khan pay tribute to London Bridge terror victims
The rise of "low-sophistication" attacks, often carried out by lone culprits - such as the Fishermongers' Hall attack and the strike on Westminster Bridge and Parliament in March 2017 - is making it more difficult for police and intelligence agencies to scupper incidents before they take place, the Tories said.
The party spokesman said a newly-elected Tory government would work with owners and operators of public venues during a consultation to develop the "best approach" to ensuring public venues were safe.
The option of bringing in legislation to force venue owners' hands in making the safety changes would be under consideration, the Tories confirmed.
A newly-elected administration would also consult on a "survivors' charter" to ensure fast access to mental health support and compensation.
Figen Murray, mother of Mr Hett who was killed in the 2017 Manchester attack, has been campaigning to make it compulsory for every venue to assess the risk of an attack, in what has been dubbed "Martyn's Law".
She said: "When Martyn died, I promised both myself and him that I would do everything in my power to stop other families going through what we were.
"Today is a real breakthrough in those efforts and I thank the Prime Minister for responding in the way he has.
"Martyn's Law won't stop terrorism - nothing can - but it will make it harder to commit. It's the sort of common-sense law that most people would think already existed."
Charlotte Dixon-Sutcliffe, whose partner David was killed in the Brussels metro bombing in 2016, said she welcomed the proposals.
The chair of Survivors Against Terror said: "We strongly welcome the commitment of the Prime Minister to support Martyn's Law and the Survivor's Charter.
"Combined these two policies will make our country safer and provide the sort of support to survivors that the British public would expect. We also welcome today's commitment from the Labour Party to support the charter."