Britain is preparing to dust off its military reserves and put tens of thousands of former troops on a longer war footing as the government confronts what ministers describe as an increasingly dangerous security environment.
New legislation will make it easier for the government to mobilise members of the strategic reserve, extending the age at which former service personnel can be called up from 55 to 65 and lowering the threshold for deployment.
The changes come as Russia continues its war in Ukraine, tensions rise over Britain’s security interests and ministers face mounting pressure to demonstrate that the UK could respond rapidly if Europe were plunged into a wider conflict.
Under the existing rules, strategic reservists can be called upon in a “national danger, great emergency or attack in the UK”. The new threshold will be “warlike preparations”, giving the government significantly greater latitude to activate the force before an actual attack takes place.
The move is one of the clearest signs yet that Whitehall is planning for the possibility that Britain could have to transition rapidly from peacetime to a wartime footing.
Pat McFadden, Cabinet Office minister, previously said the UK must “actively prepare for the possibility of the UK coming under direct threat, potentially in a wartime scenario”.
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The country was “in an era in which we face confrontation with those who are threatening security”, he told MPs, describing Russia’s war against Ukraine as “the most obvious and pressing example of this”.
The government is also preparing for a major mobilisation exercise next year involving the UK’s 95,000-strong strategic reserve. Former members of the armed forces could be contacted, equipped and mobilised in what ministers say will be the biggest test of the system for decades.
Louise Sandher-Jones, Armed Forces minister, said the strategic reserve would be exercised “on a scale we haven’t seen for a long time”.
The exercise is intended to expose weaknesses in Britain’s ability to rapidly bring former troops back into service — including a fundamental problem: knowing where they are.
Lord Robertson of Port Ellen, who led last year’s strategic defence review, previously said officials did not “know where most” members of the strategic reserve were. He urged ministers to “round up those who are available and fit and willing”.
Air Chief Marshal Sir Richard Knighton, Chief of the Defence Staff, has similarly warned that Britain lacks an “effective system of tracking and engaging” personnel who have recently left the Armed Forces.
Giving evidence to the House of Lords national resilience committee, Sandher-Jones said the exercise would test every stage of a mobilisation, from identifying personnel to putting them into the field.
“The strategic reserve is a real commitment for those that are in it and they will be exercised as part of our exercise programme next year where we will go through measures to call out, use, equip and all the rest of the procedures we’d have to go through to use the strategic reserve properly,” she said.
The reforms come against a backdrop of growing concern about the resources available to Britain’s armed forces. Labour has pledged to increase defence spending, but the government has not denied reports that Sir Richard warned the prime minister and chancellor shortly before Christmas 2025 of a potential £28bn shortfall in planned defence spending.
The strategic reserve therefore represents more than an administrative reform. It is an attempt to create a deeper pool of military manpower at a time when governments across Europe are confronting the possibility of prolonged confrontation with Russia.
For ministers, the test will be whether Britain can mobilise its reserves before a crisis becomes a war — rather than discovering its weaknesses once the fighting has already begun.
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