The prime minister has confirmed the general election will take place as planned on 8 June, despite another terrorist attack in London.
Speaking outside Downing Street, she said: “Violence can never be allowed to disrupt the democratic process.”
Political campaigning would resume in full on Monday, she said, after most parties suspended national campaigns.
Seven people were killed in the attack near London Bridge, the third terrorist attack in the UK in three months.
UKIP is the only major party to say it would not suspend national campaigning on Sunday, just days before the vote, with leader Paul Nuttall arguing that was “what the extremists would want”.
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The terrorist attack began when a white van hit pedestrians on London Bridge shortly after 22:00 BST on Saturday, before three men got out and stabbed people in nearby Borough Market.
Seven people have been killed and scores injured. The three suspects were shot dead by police.
The prime minister gave a statement after chairing a meeting of senior ministers and security chiefs in the government’s emergency Cobra committee on Sunday morning.
“But violence can never be allowed to disrupt the democratic process, so those campaigns will resume in full tomorrow and the general election will go ahead as planned on Thursday.”
The prime minister said the country must “pull together” and unite to “defeat our enemies” and said “things need to change” in the way that extremism and terrorism are tackled.
Labour Leader Jeremy Corbyn said he was “absolutely shocked and horrified” at the attack, but added: “If we allow these attacks to disrupt our democratic process, then we all lose.”
He said “as in Manchester – all communities must come together. Our strength is the strength of our community unity”.
Election campaigning was paused for three days nearly two weeks ago after a suicide bomber killed 22 people at the Manchester Arena.
Just weeks earlier on 22 March, before the election campaign began, there was another attack on Westminster Bridge, by a man who drove into crowds and stabbed a policeman – five people were killed and the attacker was shot dead by police.
BBC political correspondent Ellie Price said the last Sunday before polling day was usually one of the busiest days of the election campaign and party leaders had been expected to make campaign speeches which were now on hold.
She said the attack was likely to affect the campaign both in tone and in content and as parties would have to be careful not to be seen to be making political capital out of it, while also addressing voters’ concerns about security.
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In 2001, Tony Blair postponed plans for a general election by a month, because of the foot-and-mouth crisis.
But Brexit Secretary David Davis earlier played down reports the latest attack might mean the election would be delayed.
“On the one hand, clearly we want to respect the people who have been injured and killed, we want to pay proper respect and therefore we don’t want to carry on across it.
“But on the other hand, the people who are doing this, are doing this because they despise the freedoms we have, and those freedoms can be to go out on a Saturday night or the freedom to cast a vote”, he said.
Lib Dem leader Tim Farron said: “We must make sure we defend our country – that does not mean letting the pathetic cowards win, by us dismissing our democracy or getting rid of our freedoms.”
A spokesperson for the SNP said: “All of our thoughts are with those affected by the dreadful events in London… It is important – especially so close to an election – that the terrorists do not succeed in their attempts to undermine our democracy.”
Meanwhile Green Party co-leader Caroline Lucas said: “My heart goes out to all those affected and caught up in these horrific and despicable attacks, and my thoughts are with the victims and their families.”
Explaining UKIP’s decision to continue national campaigning, party leader Paul Nuttall said it was “time to start honouring our dead with more than just words”.
“The only guarantee that will come from our choosing to stall the democratic process again will be more attacks – it is what these cowards want us to do.”