Troops face swingeing cuts despite the PM’s pledge of a £75billion lift.
Army coffers are being raided to fund the RAF’s new Tempest fighter jet and plug a £20billion hole in MoD accounts.
Plans for a new air defence system could be put on hold, along with orders of new kit.
Army numbers are due to shrink to 74,000, down from the 82,000 in 2021.
Rishi Sunak has vowed to up Defence spending to 2.5 per cent of GDP by 2030.
Ex-Joint Forces chief General Sir Richard Barrons said: “Defence is running on fumes.”
The MoD said budgets are “routinely flexed” to deliver its priorities.
They MoD said: “We are continuing to invest in the Army, including through news tanks, armoured vehicles and artillery guns.
“Alongside this, GCAP will be our new stealth fighter jet to secure our skies for decades to come.”
“A report by the Rusi think tank warned the Tempest Global Combat Air Programme was “writing cheques that defence can’t cash”.