Britain announced on Monday that it would build 12 new attack submarines as it launched a major defence review to move the country to "war-fighting readiness" in the face of "Russian aggression" and the changing nature of conflict.

Prime Minister Keir Starmer warned that "the threat we now face is more serious, more immediate and more unpredictable than at any time since the Cold War," as he launched the review in Glasgow.

"We face war in Europe, new nuclear risks, daily cyberattacks, growing Russian aggression in our waters, menacing our skies," he added.

The Strategic Defence Review, which assesses threats facing the United Kingdom and makes recommendations, said that Britain was entering "a new era of threat".

As a result, Starmer said his government aimed to deliver three "fundamental changes".

"First, we are moving to war-fighting readiness as the central purpose of our armed forces," he said.

"Every part of society, every citizen of this country, has a role to play, because we have to recognise that things have changed in the world of today. The front line, if you like, is here."

Secondly, the prime minister insisted that UK defence policy would "always be Nato first", and finally that the UK "will innovate and accelerate innovation at a wartime pace so we can meet the threats of today and of tomorrow."

The UK has been racing to rearm on a perceived Russian threat and fears that US President Donald Trump will no longer help protect Europe.

Starmer said the review would serve as "a blueprint for strength and security for decades to come", taking into account the increasing use of drones and artificial intelligence on the battlefield.

His government pledged in February to lift defence spending to 2.5 percent of GDP by 2027 in the "largest sustained increase in defence spending since the end of the Cold War".

And despite budget constraints, it aims for spending to rise to three percent in the next parliamentary term, due in 2029. (AFP)