"I believe this is the right thing to do, because this is how we'll deliver a secure future for everyone and our country," Sunak told reporters on the campaign trail on Monday at the Chesham Football Club, just north of London.
"It's a clear plan, bold action."
Under Sunak's national service scheme, 18-year-olds will be able to choose between spending one weekend a month volunteering over the course of a year, or to take up one of 30,000 spaces to spend a year in the armed forces.
The Conservative Party said the proposal would be funded by cracking down on tax avoidance and evasion, and by diverting money from the UK Shared Prosperity Fund, which existed to reduce regional economic inequality. Interior minister James Cleverly told broadcasters there would be no criminal sanctions for skipping mandatory service but people would be compelled to do it, without providing further details.
Sunak's comments come as the country's main parties carry out campaign events to meet voters ahead of the July 4 national election, with the governing Conservatives lagging the opposition Labour by about 20 points.
The national service announcement followed Labour Party leader Sir Keir Starmer's comments on Saturday that he was in favour of allowing 16 and 17-year-olds to vote.
Sunak's decision to call a summer election left lawmakers from his own party blindsided last week, with some saying they were baffled and frustrated by the timing of the vote.
Steve Baker, Britain's minister for Northern Ireland, who had backed Liz Truss in a 2022 leadership contest to replace Boris Johnson, issued a statement on his website, criticising the policy and the lack of transparency in the policymaking process.
"Candidates are not involved and even relevant secretaries of state are taken by surprise when policy is announced. It is perhaps the worst aspect of the democratic process and I have no reason to think it is not common to the Labour Party.
"When people are doing no harm, government should leave them to work through their own virtues and vices in voluntary association with other people," Baker said.
Tory MP suspended for backing right-wing party
Meanwhile, MP Lucy Allan, who had said she would not stand in the election, was suspended from the governing Conservative Party on Monday after endorsing the candidate for the right-wing Reform UK party.
It marks another setback for Sunak, as dozens of Conservative MPs have already said they won't be standing in the July 4 election.
Allan, 59, who represented the English town of Telford and had been an MP for nine years, said on social media that she was supporting Reform's Alan Adams to be the constituency's next MP.
A Conservative Party spokesperson said she had been suspended from the party with "immediate effect," adding that "a vote for Reform is a vote for Keir Starmer."
A poll conducted by More in Common published a day after Sunak called the election on Wednesday showed 44% support for Labour and 27% for the Conservatives, with the right-wing Reform UK party at 10%.
Earlier this year, former Conservative Party deputy chairman Lee Anderson, who was suspended from the party over accusations of Islamophobia, defected to Reform, which has Brexit campaigner Nigel Farage as its honorary president.
