UK foreign policy would change under a Labour government to one that “reduces rather than increases the threat” to the country, Jeremy Corbyn is to say.
As election campaigning resumes after the attack in Manchester, the Labour leader will point to links between wars abroad and “terrorism here at home”.
In a speech, Mr Corbyn will say the “war on terror is simply not working”.
Security Minister Ben Wallace told the BBC his comments were “inappropriate and crassly timed”.
Meanwhile, Prime Minister Theresa May will chair a session on counter-terrorism with G7 leaders in Sicily, Italy, on Friday.
She is expected to focus on what can be done to deal with the threat posed by extremists online.
In London, Mr Corbyn will deliver his speech as the bigger political parties return to the campaign trail following Monday night’s suicide bombing at Manchester Arena that killed 22 people, many of them children, and injured 116.
According to pre-released excerpts from his speech, Mr Corbyn will pledge a “change at home and change abroad” if Labour wins power.
He will say that “many experts… have pointed to the connections between wars our government has supported or fought in other countries and terrorism here at home”.
“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.”
Mr Corbyn, who opposed UK military intervention in Iraq and Afghanistan, and voted against strikes in Libya and Syria, will say: “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.”
He will add: “No government can prevent every terrorist attack. If an individual is determined enough and callous enough, sometimes they will get through.
“But the responsibility of government is to minimise that chance – to ensure the police have the resources they need, that our foreign policy reduces rather than increases the threat to this country, and that at home we never surrender the freedoms we have won and that terrorists are so determined to take away.”
Shadow international trade secretary Barry Gardiner said there was no “simple causal relationship” between foreign intervention and terrorism, but “we need profoundly to reassess the way in which there are linkages”.
Speaking on the BBC Radio 4’s Today programme, Mr Gardiner said it was “undeniable that young radicalised men from this country have used the excuse of British foreign policy and the chaos that has often resulted”.
He used the example of British foreign policy in Libya – where the Manchester attacker Salman Abedi is known to have visited – as one that had failed to deal with the aftermath of military intervention.
Libya is in “complete chaos”, he said.
But Conservative Security Minister Ben Wallace told Today Mr Corbyn’s timing was “appalling”.
“We have to be unequivocal, that no amount of excuses, no amount of twisted reasoning about a foreign policy here, a foreign policy there, can be an excuse. The reality is, these people hate our values.
“The way to see that off is not to feed excuses, not to allow these people – that right now our police are trying to find – to give them any cause to think: Well I’m slightly justified.
“They are not justified.
“In this country it’s democracy, the rule of law, equality that we stand for and we absolutely reject terrorism – and so should Jeremy Corbyn – on all fronts.”
Mr Corbyn is also expected to say that a Labour government would fund more police and ensure the security services had sufficient resources “to keep track of those who wish to murder and maim”.
Labour’s former home secretary, Charles Clarke, told BBC Two’s Newsnight that Mr Corbyn was “simply wrong” about the war on terror.
He said the primary motive of radicals was not the UK’s foreign policy but was instead “about the destruction of all the core elements of our society”, adding: “It’s about a totally opposed vision of what society should be.”
Former Lib Dem leader Lord Ashdown said: “Some political leaders have sought to politicise the events of the week, but now is not the time, and this is not the event, to seek political advantage.”