Gavin Williamson has used a speech in the United States to deliver a thinly-veiled warning to Theresa May.
The defence secretary said his job was to make sure the UK can “develop and if necessary deploy hard power”.
“Britain is a major global actor. We have always been a tier one power and we always will be a tier one military power,” he said.
The prime minister has repeatedly avoided using the term “tier one” when asked about her commitment to defence spending.
May has instead said she wants the UK to remain a “leading military power”.
Tier one powers are countries seen as those with a full range of globally deployable military capabilities.
Labour seized on May’s decision to dodge using the phrase as evidence she may cut the military.
In a speech to the Atlantic Council in Washington DC, Williamson also offered a strong defence of Brexit.
“Please never underestimate my nation,” he said. “The UK remains a great power.”
He added: “While Britain is leaving the EU, we are clear about our role and place in the world.
“Brexit is Britain’s moment. Britain’s moment to look up, be more ambitious, redefine our place in the world.
“In some ways the EU has limited our vision. Discouraged us from looking to the horizon. Now we are being freed to reach further and aim higher.”