Defence Secretary Ben Wallace has said the country has to start "investing in defence properly" as he defended the UK military's readiness for war.
He said the army had been "hollowed out" over 30 years and the Ukraine war had "exposed our vulnerabilities".
Mr Wallace said he wanted a bigger budget, amid reports he is asking for a £10bn rise.
UK and European officials have raised concerns over the state of the British armed forces.
Malcolm Chalmers, a British defence expert who advises MPs on national security, told the BBC the UK military "would run our of ammunition in days if we faced a war, such as the ones the Ukrainians are facing right now".
When asked his reaction to those concerns, Mr Wallace said the UK government was going to spend £34bn on modernising the army.
The defence secretary said the UK military was "not any less ready than others", but added: "We just need to make sure we get back to spending on our defence properly."
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Mr Wallace spoke to the BBC from Brussels, where he is meeting Nato defence ministers for a summit at which Ukraine will top the agenda.
Calls for increased spending on defence have been growing ahead of an expected spring offensive by Russia in Ukraine, and warnings about the threat from China after a suspected spy balloon was shot down over the US.
When asked if he was requesting £10bn more in the upcoming budget, Mr Wallace said the Ministry of Defence - like all other departments - had been affected by rising costs.
But he said he would "make the case to the Treasury that I will need some money to insulate myself".
Despite inflation and military budget cuts in the past, the UK has been one of the biggest supplier of arms to Ukraine in its war against President Vladimir Putin's invading forces.
The UK is set to become the first nation to start training Ukrainian pilots on Nato-standard aircraft, but the government has indicated that lending jets to Kyiv is a long-term prospect.
At the end of this year, the UK will be taking over the leadership of Nato's Response Force (NRF) from Germany.
Mr Wallace said reports about Nato chiefs asking Germany to stay in charge of the organisation's rapid-reaction force were "just bollocks".
The defence secretary said: "I mean, to be honest, the simple reality is Nato leadership did not approach anybody. We are taking over the NRF as scheduled and it's interesting that story is based on a source on a German website I've never heard of."
Downing Street has confirmed that Prime Minister Rishi Sunak will travel to Munich this weekend, joining fellow world leaders for a conference on international security.