Rishi Sunak and Sir Keir Starmer have come to blows over the Conservatives’ plan to introduce national service for teenagers.

Labour leader Sir Keir said the Conservatives’ first major policy announcement of the election campaign would amount to a “teenage Dad’s Army”, in reference to the popular 70s sitcom about a hapless group of men who were ineligible for military service.

But the prime minister defended his plan for 18-year-olds to serve in the military for a year or do mandatory volunteering, saying it is “absolutely the right policy at the right time”.

Sir Keir called the policy “desperate”.

“All this spinning round and round, it’s symbolic of the chaos and the instability,” the Labour leader added.

“You’ve seen that again over the past few days, the desperation of this national service policy, a sort of teenage Dad’s Army, paid for, I kid you not, by cancelling levelling up funding and money from tax avoidance that we would use to invest in our NHS.

“I think they are rummaging around in the toy box to try and find any plan that they can throw on the table. I don’t think it’ll work.”

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Mr Sunak insisted the plan would “give young people skills and opportunities for life”.

He added: “It’s going to foster a culture of service that will make our society more cohesive. And it’s going to strengthen our country’s resilience and security.

“So I think it’s absolutely the right policy at the right time.”

He dismissed suggestions mandatory national service was an un-conservative policy, and said: “I believe this is the right thing to do because this is how we’ll deliver a secure future for everyone and our country.”

Sir Keir, in his first major speech of the campaign, said the Conservatives were planning to take money from the levelling up fund to pay for the national service policy, which shows “they’ve completely abandoned the project they put before the electorate in 2019”.

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He accused Mr Sunak of governing to appease sections of the Tory party, not for the whole country, and said the government’s Rwanda policy was evidence of that.

Placing security at the heart of his speech, Sir Keir said the scheme to send asylum seekers to Rwanda was part of Mr Sunak’s “gimmicks and gestures”.

“He never believed in it. He knew it wouldn’t work. He said that they tried to stop it when he was chancellor, but he was too weak to stand up to his party,” he said.

“He caved in, and now he’s gone through and it’s cost £600m. And now has called an election before it can be tested. Weakness upon weakness.”

The Labour leader admitted he was “not against third country processing” and it has been successful in places such as Afghanistan but said there was a difference in processing people in a different country and “simply deporting people to Rwanda”.

During his speech on Monday, Sir Keir went over the policies Labour is offering up to voters as he tried to persuade them he has turned the Labour Party around from its Jeremy Corbyn era.

But Mr Sunak accused the Labour leader of having “no plan, no ideas”.

“We’ve had another speech from Keir Starmer, another half hour speech. Not a single new idea. He’s taking the British public for granted,” he said.

“I’m the one that’s putting bold ideas on the table. I’m the one that’s got a plan, and that’s how we’re going to deliver a secure future for everyone.

“And as I said his approach is to take people for granted. He’s got nothing to say, no plan, no ideas.”