ComRes said the lead of May's Conservatives had fallen to 12 percentage points in an online poll carried out May 24-26, from 18 percentage points in a comparable poll on May 13.
She said it was "absolutely right" political campaigning had ceased in the aftermath of the Manchester Arena bombing - but also said it was important politicians were again back out on the election trail.
Mr Corbyn said he was "concerned" that Army troops have been drafted in to protect certain potential targets, blaming the move on a shortage of police officers due to Tory cuts.
The Labour leader was accused of having "basically supported the armed struggle for a united Ireland, but now you want to be Prime Minister you have to distance yourself from it".
The Labour leader said Trident renewal would go ahead because MPs had voted for it and the Labour Party backed it.
Denied Labour was planning "a massive spending binge", insisting 95 per cent of people would not pay higher taxes.
"What I said was we have to be serious about the situation that exists in Libya and other countries and a sensible government would also be looking at the worldwide situation - ungoverned spaces in Libya are a danger to all of us".
Speaking at the G7 Summit in Sicily, Theresa May said: "Jeremy Corbyn has said that terror attacks in Britain are our own fault".
"We've seen it in America in the Twin Towers attack, we've seen it in Sweden and Belgium who haven't take the same foreign policies decisions as us".
He suggested this could indicate apathy to politics or that the party leaders "simply aren't charismatic enough". Either Jeremy Corbyn negotiating Brexit just 11 days after the election, or Theresa May. We have to have a government that knows what its approach will be, and that has the strong hand to take into that negotiation process'.
On March 22 this year, two months to the day before the Manchester terror attack, Khalid Masood killed five people, including Pc Keith Palmer, in a auto and knife attack in Westminster.
Mr Corbyn stressed the link between foreign policy and terrorism could not "remotely excuse" the actions of terrorists such as Manchester suicide bomber Salman Abedi.
The Prime Minister has a 45 to 28 per cent advantage over the Labour leader on who makes the best prime minister.
The Tories started the election campaign with more than 20 points lead. She retains an eight-point advantage among working-class C2DE voters.