Keir Starmer has warned it is a "mistake" to think the only threats Britain faces are far off - saying the British homeland faces danger "on a daily basis".
Asked by The Mirror about an alarming dossier that said Brits must "actively prepare" for war, he voiced his alarm over "sophisticated" attacks on the UK mainland. The Prime Minister announced that security spending will be ramped up dramatically in a "decisive message to aggressors".
He stated: "It is a mistake to think that the only threat we face is external and far off. We do face threats at home all of the time on a daily basis.
"There are cyber attacks that have to be dealt with and all being dealt with. They are ever more sophisticated. We need to have the capability to deal with them."
He said that security services have dealt with "many attempts to penetrate our systems", and added: "The fact that they are very good shouldn't detract from the fact that that is a real threat to our country."
The PM stated that Europe has made a "fundamental shift in its posture" by ramping up its focus on security in the face of new threats. He said NATO members have agreed to raise spending to 5% within a decade, and vowed that the UK will reach 4.1% by 2027.
The Prime Minister said he will oversee the "biggest strengthening" of the UK's nuclear deterrence in three decades after confirming RAF pilots will get nuke-capable jets.
Mr Starmer, speaking at The Hague in the Netherlands, also shot down suggestions that US President Donald Trump is not committed to defending his allies. There has been growing alarm after Trump gave a less-than-convincing response when asked if he would honour Article 5 of the NATO pact - which sets out that an attack on one member is an attack on all.
Speaking to reporters in Washington on Tuesday, Trump said it would "depend on" the definition of the clause. But Mr Starmer said the President is a reliable friend to the UK., claiming the relationship between the two nations is "as strong now as it's ever been".
The PM was seen in deep conversation with the US President during the summit. He praised the US for launching missiles at Iranian nuclear cites.
Mr Starmer said: "It has been UK policy for decades that Iran can never be allowed to have a nuclear weapon.
"Over the weekend, the US moved to alleviate that threat. Now we need to make sure that the ceasefire holds and seize this opportunity to stabilise the region and get Iran back around the negotiating table with the US.
Ultimately, this is how we will ensure a complete, verifiable and irreversible end to Iran’s nuclear program. And we’re using every diplomatic lever to support this effort."
Hailing the strength of the NATO alliance, he said: "This is the moment to unite, for Europe to make a fundamental shift in its posture, and for NATO to meet this challenge head on.
"And that is what we have done today."
Mr Starmer's remarks came a day after a bleak National Security Strategy said the years ahead will test the nation - with a World War Two-style spirit needed to deal with growing threats.
It said tackling the danger of nuclear weapons will be "more complex than it was even in the Cold War". The document went on to warn that major powers like China and Russia are seeking to gain an upper hand in "outer space, cyberspace, the deep sea, and at the Arctic and Antarctic poles".
The document says: "The years ahead will test the United Kingdom... The direction it takes – and the decisions we take – will reverberate through the decades.
"We will need agility and courage to succeed, but we should be optimistic. We remain a resolute country, rich in history, values and in our capabilities. But most of all, there is the determination of the British people themselves. After all, we do not need to look too far into our history for an example of a whole-of-society effort, motivated by a collective will to keep each other safe.
"We can mobilise that spirit again and use it both for our national security and the rebuilding of our country."
Defence Secretary John Healey said the PM trusts that Trump's America would come to the aid of NATO allies. He told Times Radio: "Do I trust President Trump and the US's commitment to Article 5? Yes.
"So does our Prime Minister, and he does because in the Oval Office on his visit to the White House, President Trump gave him that commitment in public."
Mr Starmer has called on Israel and Iran to maintain the pause in hostilities. In a conversation with the French and German leaders at on Tuesday, he "reflected on the volatile situation in the Middle East," according to a Downing Street spokeswoman.
The leaders agreed that "now was the time for diplomacy and for Iran to come to the negotiating table", the spokeswoman added. It comes as intelligence reports in the US suggested that the American attack on Iran's nuclear programme over the weekend have only set it back by a few months, rather than destroyed it as Donald Trump previously suggested.