Outgoing UK Prime Minister Keir Starmer announced Tuesday that Britain would spend almost £300 billion ($397 billion) over the next four years to modernise its armed forces amid rising threats.

Starmer, expected to leave office next month after losing the support of Labour MPs, announced that the overall defence budget would increase by £15 billion over the next four years to almost £300 billion, as he launched his long-awaited 10-year Defence Investment Plan.

"Last year I made the decision in the national interest to reprioritise aid spending towards defence and achieved the biggest uplift in defence spending since the end of the Cold War," Starmer said in a speech.

"That was the right choice because the world has changed. National security is economic security.

"Today we uplift defence spending further. An additional £15 billion worth of funding by ... reprioritising spending across government."

The plan includes more than £5 billion for drones and autonomous systems over the next four years, the Ministry of Defence (MoD) said earlier in a press release.

The announcement follows months of wrangling within Starmer's Labour government over the resources required to modernise the UK's armed forces in the face of rising threats, including from Russia.

Two defence ministers quit earlier this month in a row over spending proposals, including defence secretary John Healey who said the plans risked making Britain "less safe".

The UK's pledge came as US President Donald Trump has repeatedly urged NATO allies to spend more on defence and become less reliant on Washington for security.