An audience member at a Conservative conference event addressed by Boris Johnson joked that Jeremy Corbyn should be hung, footage shows, amid growing concerns over the use of violent language in politics.
Video from a reception hosted by the Democratic Unionist Party (DUP) at the gathering in Manchester also shows the Labour leader being branded a "traitor".
The event was hosted by DUP leader Arlene Foster and the party's parliamentary leader, Nigel Dodds, and attended by Mr Johnson.
In a clip from the prime minister's speech, posted by Sky News, Mr Johnson can be heard suggesting that Mr Corbyn should be invited to "step quietly into a figurative rocket" and "into orbit where he belongs".
But an audience member calls out to suggest that Mr Corbyn should instead be told to enter "traitor's gate", while another simply shouts "a noose". The comments are greeted with laughter.
In the video, Mr Johnson tells the audience: "Northern Ireland is part of the union with Great Britain, it is part of the United Kingdom, and let it remain so forever.
"Let's get Brexit done, let's get Stormont up and running again, and then let's get ready together to invite that threat to the union, Jeremy Corbyn, to step quietly into a figurative rocket and then let us send him into orbit where he belongs."
Mr Johnson also praised what he called the "seething sea of faces" at the event, adding: "I know that we unionists are in good heart, and that we will prevail."
And he agreed with an audience member who suggested that a government led by Mr Corbyn would be "humbug", saying: "Yes it would". The mention of the Labour leader was greeted with boos and cries of "shame on him".
The incident comes days after Mr Johnson was criticised for his language after dismissing MPs' warnings about intimidation and death threats as "humbug".
The prime minister was also urged to stop referring to a law blocking a no-deal Brexit as the "surrender act", while a member of his team was quoted in a Sunday newspaper suggesting that anti-Brexit MPs were "collaborating" with EU countries.
Earlier this week, opposition party leaders and other senior parliamentarians signed a pledge to "moderate" their language.
It said: "We all feel that those in leadership positions have a particular duty to weigh their words carefully, bearing in mind that there are stark divisions across the country on Brexit.
"The right of a member to personal safety is absolute and unconditional."
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