Jeremy Corbyn has been accused of being "crass" and totally inappropriate" for linking Britain's involvement in military interventions abroad with UK terrorism.
In a thinly-veiled attack on the Conservatives, Mr Corbyn said it is the "responsibility" of all governments to minimise the risk of terror by ensuring their foreign policy does not increase the terror threat to the UK.
Theresa May hit back after the Labour leader's speech, suggesting that he was saying that attacks such as the Manchester Arena suicide bombing were "our own fault".
"I want to make one thing very clear to Jeremy Corbyn," she said. "It is that there can never, ever be an excuse for terrorism. There can be no excuse for what happened in Manchester."
Security Minister Ben Wallace also savaged the Labour leader's stance as campaigning resumed four days after the Manchester terror attack.
Mr Wallace, who has responsibility for counter-terrorism matters, said Islamist terror was aimed at the British way of life rather than a response to foreign military interventions.
He told BBC Breakfast: "He needs to get his history book out, to be brutally honest."
The Conservative minister added: "These people hate our values, it's not our foreign policy they go to war with us about."
The Tory minister went on to say that Corbyn's comments are "totally inappropriate and crassly timed" while the police operation in Manchester was ongoing and people were being treated in hospital following Monday's atrocity.
"Now is not the time to decide to use this event to attack foreign policy decisions that may or may not have been made," he said.
Former Liberal Democrat leader Lord Ashdown also criticised Mr Corbyn's speech, saying: "Now is not the time, and this is not the event, to seek political advantage.
"The families of victims in Manchester have a right to expect political parties to respond with restraint and sensitivity to these unpardonable crimes.
"There will be a moment when we will want to look at the policy implications of what has happened, but that should not be in the shadow of these terrible events when the nation should stand together."
Mr Corbyn, who opposed Britain's military involvement in wars in Iraq and Afghanistan as well as air strikes against terrorist targets in Syria, pledged to take a new approach towards countries which foster extremist violence.
And he insisted it is time to recognise the West's "war on terror" is not working.
"Many experts, including professionals in our intelligence and security services, have pointed to the connections between wars our government has supported or fought in other countries and terrorism here at home," Mr Corbyn said.
"That assessment in no way reduces the guilt of those who attack our children. Those terrorists will forever be reviled and held to account for their actions.
"But an informed understanding of the causes of terrorism is an essential part of an effective response that will protect the security of our people that fights rather than fuels terrorism.
"We must be brave enough to admit the 'war on terror' is simply not working. We need a smarter way to reduce the threat from countries that nurture terrorists and generate terrorism."
Mr Corbyn also promised a Labour government will provide the funding that security agencies and emergency services need.
Making a direct reference to the Manchester attack, he said the "solidarity, humanity and compassion" shown on the streets of the city in the aftermath of the bombing would be the values which guide his government in office.
All the major parties resumed national campaigning on Friday for the June 8 election.
Mrs May will not be involved as she is in Italy for a summit of the G7 group of industrialised nations.