The Conservatives would reinstate the two-child benefit cap and use the savings for a wide-ranging spending splurge on defence in what Kemi Badenoch said would be “the biggest peacetime programme of rearmament in our country’s history”.

Speaking at a defence conference in London, the Tory leader criticised the government for Britain’s “lack of readiness” for war, which has been exposed by recent world events.

Badenoch said the UK needed to “reassert” itself as a global power and committed the Tories to “the largest net increase in British troops under any government since the second world war” if they returned to power at the next general election.

The pledge would involve recruiting 6,000 full-time soldiers and 14,000 reservists. The Tories say they could raise £20bn towards the venture by reinstating the two-child benefit cap and reallocating money earmarked for net zero projects.

Badenoch said: “We must look beyond this conflict in the Middle East and develop the resources we need to respond to this new era of threats. We have a huge amount of work to do to reassert Britain as a power in the world.

“First and foremost, we must rearm. If our military is strengthened, our hand is strengthened. We must undertake the biggest peacetime programme of rearmament in our country’s history.”

Badenoch has been accused by Labour of prematurely stating that the UK should have joined Donald Trump’s war with Iran, which has grown into an international crisis that the president has struggled to end.

Speaking of the UK-US relationship, which has become strained owing to Keir Starmer’s unwillingness to play an offensive role in the conflict, Badenoch said she found Trump’s public criticism of the UK prime minister “disconcerting”.

She said: “I do find the nature of [Trump’s] public remarks very disconcerting, because everybody’s watching. It’s not just on social media and the citizens of our respective countries.

“Iran is watching, China is watching, Russia is watching … what they see, and that is being reinforced, is the weakening of western bonds, and that is not something we should allow.”

Starmer has defended his decision to limit British involvement in the Iran war to “defensive” action, refusing to allow the US access to launch widespread attacks from its bases. The prime minister has said he acted in the national interest by limiting British involvement in the war to “defensive” action, after US-Israeli strikes on Tehran and Iranian retaliation against US allies in the Gulf.

The Labour government has said it is committed to spending 2.5% of GDP on defence by 2027, increasing to 3% in the next parliament. However, it is under pressure to publish a defence spending plan, which was first promised last autumn, amid reports of tensions between the Ministry of Defence and the Treasury.

Labour ministers have argued that they have inherited years of underinvestment from the previous Conservative government and have accused the party of “hollowing out” the armed forces.

Defence spending under the Conservatives fell by 22% between 2010 and 2017, but rose steadily after this point. It has now returned to 2010 levels.