Britain is planning a major drill to mobilise its army of veterans as it prepares for war.
Louise Sandher-Jones, the Armed Forces minister, revealed the exercise would take place next year as part of a wider operation to test the UK's resilience.
A cohort from the "strategic reserve" of 95,000 retired troops will be called into action "on a scale we haven't seen for a long time", Ms Sandher-Jones said.
The planned operation was disclosed as the UK seeks to put itself on a war footing amid fears that it could be dragged into a conflict with Russia by 2030.
Moscow has already threatened to attack British factories building drones for Ukraine. Western intelligence officials have also warned Vladimir Putin is waging a sabotage campaign that could lead to a "miscalculation" that triggers a war.
Andy Burnham, the Prime Minister, is facing increased scrutiny over Labour's military spending plans after it emerged that John Healey, the Chancellor, would fail to commit to spending 3 per cent of gross domestic product on defence by 2030 in his first Budget next month.
Government cost-cutting measures have already led to training being cancelled for some full-time soldiers in the Army's 3rd Division, with live-firing drills reportedly suspended until April to save money.
It is unclear how many of the strategic reserve will join next year's drill, which The Telegraph understands is still being finalised. However, the drill is expected to test how successfully the veterans can be called out and then equipped in a crisis.
Revealing the drill on Monday, Ms Sandher-Jones said: "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."
The retired Army officer added that the network of former troops would have a crucial role in defending the country, but she noted it had been "a lot less used in my lifetime".
"When I left the Army and found out I was in the strategic reserve, I just filed it away and never thought about it again. We are definitely no longer in that space," she told the House of Lords' national resilience committee.
The operation will be a major test for the Government after it was revealed that it had lost track of much of the network of retired soldiers and officers from the Army, RAF and Navy.
Under existing law, all former regular and reserve officers retain recall liability for life. The Ministry of Defence (MoD) must also maintain contact with former military staff in the first six years after they leave full-time service.
This group is different to the reserve forces, which comprise personnel who volunteer part-time to serve in the military. Currently, there are almost 32,000 active reservists across the three wings of the Armed Forces.
In January, it was announced that the age military veterans could be mobilised would rise from 55 to 65.
The measures are being introduced as part of a new Armed Forces Bill, which will also result in the threshold being lowered for when retired soldiers can be called upon.
Under existing rules, the pool of retired soldiers can be called upon in the event of "national danger, great emergency or attack in the UK".
However, as part of these changes this will be lowered to "war-like preparations", which is already the threshold for reservists who have recently left the Armed Forces. This will come into effect in spring next year.
Lord Robertson of Port Ellen, who led the strategic defence review last year, previously claimed that defence officials didn't "know where most" of these reservists were, and called on the Government to "round up those who are available and fit and willing".
Air Chief Marshal Sir Richard Knighton, the Chief of the Defence Staff, also warned Britain did not have an "effective system of tracking and engaging" those who have recently left the military.
Meanwhile, the Army has shrunk to its smallest number in more than 200 years, with about 71,000 full-time, fully-trained troops ready to deploy to the front line.
There are early signs that the atrophy may be coming to an end, however. The MoD announced last week that more people had joined the full-time forces than had left it for an entire year for the first time since 2021.
The gains are still modest, though, with the Army having increased by 170 personnel, the RAF by 840 and the Navy and Royal Marines by 440.
(0)Comments