David Cameron has accused Jeremy Corbyn of being driven by a "security-threatening, terrorist-sympathising, Britain-hating ideology".
The prime minister's no-holds-barred attack won him a long standing ovation at the Conservative party conference in Manchester. Senior Tories have been careful in recent weeks not to make personal attacks on Labour's new leader.
But Cameron did not hesitate in criticising Corbyn and his policies. He said bluntly: "You only really need to know one thing: He thinks the death of Osama bin Laden was a 'tragedy'.
"No. A tragedy is nearly 3,000 people murdered one morning in New York. A tragedy is the mums and dads who never came home from work that day. A tragedy is people jumping from the towers after the planes hit.
"My friends – we cannot let that man inflict his security-threatening, terrorist-sympathising, Britain-hating ideology on the country we love."
Tory delegates got to their feet to applaud Cameron, while Conservative press officers clapped and cheered in the media room.
But a spokesperson for Jeremy Corbyn hit back: "The fact that David Cameron used his speech to make personal attacks on Jeremy Corbyn are a sure sign that he is rattled by the re-energisation of the Labour party.
"With cuts to tax credits and a continued failure on housing, his claim that the Conservatives are the party of working people is being exposed."
Footage emerged in August of an interview Corbyn gave to Iranian national television four years ago in which he suggested that bin Laden's death was a "tragedy upon a tragedy".
Corbyn, who was then a backbencher, told Press TV's The Agenda programme: "There was no attempt whatsoever that I can see to arrest him and put him on trial, to go through that process. This was an assassination attempt, and is yet another tragedy, upon a tragedy, upon a tragedy.
"The World Trade Center was a tragedy, the attack on Afghanistan was a tragedy, the war in Iraq was a tragedy. Tens of thousands of people have died. Torture has come back on to the world stage … Can't we learn some lessons from this?"
Emily Ashton is a senior political correspondent for BuzzFeed News and is based in London.
Contact Emily Ashton at firstname.lastname@example.org.
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