The prime minister said Britain has "generations of young people who have not had the opportunities they deserve" as he claimed the radical measure would help unite society in an "increasingly uncertain world". 

In future, 18-year-olds would be given a choice between a full-time placement in the armed forces for 12 months or spending one weekend a month for a year volunteering in their community, the Tories said. 

In an apparent pitch to older voters, the Conservative Party said this could include helping local fire, police and NHS services as well as charities tackling loneliness and supporting elderly, isolated people. 

Sunak is seeking to draw a dividing line with Labour on global security following his pledge to raise defence spending to 2.5 per cent of gross domestic product by 2030. 

Heightening his attack on Saturday, Sunak said voters would be left "at risk" with Labour's Keir Starmer in Number 10 because Britain's enemies would notice that he "doesn't have a plan". 

Teenagers who choose to sign up for placement in the forces would "learn and take part in logistics, cyber security, procurement or civil response operations,” the Tories said. 

The Conservatives said they would establish a royal commission bringing in expertise from across the military and civil society to design what they described as the "bold" national service program. 

A Labour Party spokesperson called the announcement "another desperate unfunded commitment from the Conservative Party which already crashed the economy and sent mortgages rocketing.

With AAP.