Theresa May is to urge Nato allies to take on a greater role in the fight against terrorism in the wake of the Manchester bombing.
Attending an alliance summit in Brussels on Thursday, the Prime Minister will say they need to show the same resolve in countering terrorism as they do in responding to the threat from Russia.
With President Donald Trump attending his first Nato summit, Mrs May will back a call by Secretary General Jens Stoltenberg for the alliance to join the US-led counter-Islamic State (IS) coalition.
The Prime Minister will express her gratitude for the support of Nato allies following the "callous and cowardly" attack in Manchester which she will say was "all the more sickening for the way it targeted innocent and defenceless children and young people".
"A strong capable and united Nato is at the heart of the security of each and every one of our nations. Our unity in responding to common threats is our most potent weapon," she is expected to say.
"We must redouble our resolve to meet the threats to our shared society, whether from terrorism or from Russia."
Following the meeting, Mrs May will fly to join other leaders from the G7 group of leading industrialised nations for their annual summit, being held in Taormina, Sicily.
However, a senior Government source said she was expected to cut short her stay in the light of events in Manchester, returning to the UK on Friday evening, missing the second day on Saturday.
Earlier, Mr Stoltenberg expressed confidence that the allies would agree to join the counter-IS coalition. The move is, however, seen as largely symbolic as he made clear they would not be taking part in combat operations.
"It's totally out of the question for Nato to engage in any combat operations," he said.
Nato already provides Awacs surveillance planes to support anti-IS operations as well as training officers in Iraq.
Nevertheless, a UK Government sources said that for the alliance to sign up to the coalition would sent a "strong political message" of its commitment to the counter-IS cause.
The source said that would also enable the alliance to build up the capabilities of partner states in the region such as Jordan where Britain is volunteering to run a pilot project co-ordinating counter-terrorist action.
At the same time, Mrs May will say they need to do more to address the root causes of terrorism and the propagation of "poisonous ideology", particularly online.
The Prime Minister will also back calls by Mr Trump for other allies to take on a greater share of the burden of collective defence - a key demand of the US President.
She will emphasise that the UK - along with the US - remain among just five of the member states to meet the alliance target of spending 2% of GDP on defence.