Starmer's warning comes a day before he publishes a major review of Britain's military capabilities. The UK and countries across Europe are rapidly trying to boost their defence industries after US President Donald Trump said the continent had to take more responsibility for its own security.
"We are being directly threatened by states with advanced military forces, so we must be ready to fight and win," Starmer wrote in an article for The Sun on Sunday newspaper, citing as examples Russia's invasion of Ukraine and its co-operation with Iran and North Korea.
Defence Minister John Healey announced the munitions investment program in a BBC interview on Sunday, describing it as "a message to Moscow" as well as a way to stimulate the country's sluggish economy.
The Ministry of Defence said it would procure up to 7000 long-range weapons, built in Britain. The extra investment means Britain will spend around six billion pounds ($A12.5 billion) on munitions in the current parliament, the Ministry of Defence said.
The Sunday Times reported the government wanted to purchase US-made fighter jets capable of firing tactical nuclear weapons. The defence ministry did not immediately respond to a request for comment on the report.
The Strategic Defence Review, commissioned by Starmer's Labour government shortly after it won power in July 2024, will set out the threats Britain faces and the military equipment and services needed to meet them.
Starmer has already committed to raise defence spending to 2.5 per cent of GDP by 2027 and target a three per cent level over the longer term.
The government has already announced a billion-pound plan to invest in artificial intelligence that can be used to enhance battlefield decision-making, and has separately committed to spend an extra 1.5 billion pounds ($A3.1 billion) to tackle the poor state of housing for the country's armed forces.
