Julie Hambleton, whose teenage sister Maxine was among 21 people murdered in the 1974 Birmingham pub bombings, said his support for the IRA made her "blood boil", adding that he was unfit to be Prime Minister.
UK Labour Party leader Jeremy Corbyn rejected suggestions that he was a pacifist, insisting on Friday that he would approve military action as a last resort.
Opinium's poll showed the Labour leader trailed Theresa May by 24 points, with just 23 per cent believing he would make a good Prime Minister.
In a reference to the "coalition of chaos" attack used against him by the Tories, Mr Corbyn warned that Mrs May "seeks to build a coalition of risk and insecurity with Donald Trump". "So no more hand holding with Donald Trump".
And he admitted Labour had a "mountain to climb" before voting day, although the party is determined and the final manifesto, a draft of which was leaked on Wednesday, contains "terrifically exciting" ideas.
"I am not a pacifist".
However, the Labour leader came unstuck when quizzed by journalists about whether he would be prepared to use the country's Trident nuclear deterrent.
Setting out his party's stance on defence and security, Mr Corbyn branded the current war on terror as a "failure" adding that the world had become more unsafe as a result.
"Britain deserves better than simply outsourcing our country's security and prosperity to the whims of the Trump White House", he said, adding that while he wanted a strong relationship with the U.S, he would also speak his mind.
In a series of barbed comments aimed at the shadow home secretary - who struggled in a radio interview over the cost of her policy on police recruitment - Mrs May said: "Labour cannot add up".
Mr Corbyn, a veteran opponent of nuclear weapons and military action, will attempt to reassure voters sceptical about his anti-war record.
"We will announce our policies in our manifesto, which is our plan to transform Britain for the many, not the few", a spokesperson for Corbyn said.
"It's an extraordinary question when you think about it".
"If circumstances arose where that was a real option, it would represent complete and cataclysmic failure. Deep cuts have seen the Army reduced to its smallest size since the Napoleonic Wars", he said.
She said that next week's Conservative manifesto will set out "five great challenges" facing the United Kingdom which she will tackle over the coming five years.
"I will be reaching out to those who have been abandoned by Jeremy Corbyn and let down by government for too long".