Britain must begin preparing for mass mobilisation now or risk being caught unprepared, a chilling new report has warned.
The Rusi think-tank argues military planners need to expand recruitment plans at once - while "the window of orderly action is limited".
Authors say Britain's land forces will need to double in size to counter the growing threat posed by Russia.
The findings are expected to cause concern in Whitehall, where defence officials face accusations of prolonged indecision over military spending.
Rusi is calling on ministers to act before an emergency occurs rather than waiting until crisis strikes.
Paul O'Neill, a senior associate fellow at Rusi, told The Sun the ideal time to have started ramping up Britain's mobilisation plans would have been "10 years ago".
"The next best time is now," he said.
Mr O'Neill urged the Government to expand its defence capacity without delay.
"We need to grow the capacity to absorb more reserves, then bring more in," he said, pointing to the single-centre system for calling up and training forces, warning it would choke under pressure.
Britain's land forces will need to double in size to counter the growing threat posed by Russia, the report says
|MINISTRY OF DEFENCE
"Is it going to be too late?" he questioned. "I don't know."
"We have a tradition in the UK of doing these things too late.
"Mobilisation for the First World War and Second World War - typically, it has been late and I can absolutely understand why.
"Governments do not want to commit to economic costs of mobilisation where they are not confident it is absolutely necessary."
"So I am not trying to be tough on politicians in this regard. But I think we need to start the growth journey now."
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Rusi is calling on ministers to act before an emergency occurs rather than waiting until crisis strikes
| PAThe British Army currently has 73,790 regular soldiers, a sharp decline from 153,000 at the Cold War's end following years of underfunding.
Volunteer reserves number 31,900 personnel, representing 17.5 per cent of total Armed Forces strength.
Strategic reservists are estimated at around 90,000, though this figure is considered unrealistic for actual recall due to overlapping skills across sectors.
The report acknowledges that conscription should be considered - but concludes it remains impractical for npw.
It said: "While meriting consideration, conscription is currently unrealistic, not least because society is not ready for this, and the armed forces could not absorb or adequately train any significant conscript intake or get much military utility from them."
Rusi recommends following France's model of small-scale selective conscription, announced by Emmanuel Macron last year
| GETTYRusi recommends following France's model of small-scale selective conscription to test how such a system might function.
The French use a "hybrid army model" which brings together national service youth, reservists, and the active army.
A Ministry of Defence spokesman said: "We are reversing the four-year decline in Armed Forces recruitment, with more personnel now joining than leaving for the first time since 2020, and female applicants to the Army are at the highest level in five years."
"We are taking decisive action on recruitment and retention including giving personnel the largest pay rise in decades, turbocharging improvements to defence housing, scrapping 100 outdated recruitment policies, speeding up the process for those wishing to rejoin the military and appointing the first-ever Armed Forces Commissioner to improve service life."
"The delivery of the first-of-its-kind Strategic Defence Review is backed by the biggest sustained increase in defence spending since the Cold War with £270billion across the Parliament ensuring no return to the hollowed out Armed Forces of the past."