Defence cuts could put up to 30,000 military jobs at risk during the next parliament, a defence thinktank has warned.
A report by the Royal United Services Institute (Rusi) said it was inevitable that Britain's defence spending would drop below the Nato target of 2% of GDP to 1.95% in the face of continuing austerity cuts.
Rusi warned the British Army was likely to bear the heaviest cuts, leaving the armed forces with a combined strength of just 115,000 by the end of the decade.
Even if defence spending is given the same level of protection being promised to health and schools, it said the forces are still likely to have lost 15,000 jobs over the course of the next parliament.
Plans for equipment spending could also be affected, with the need to renew the Trident nuclear deterrent leaving other programmes vulnerable.
The Rusi paper, written by research director Professor Malcolm Chalmers, said maintaining the Natio spending target would require an extra £3 billion of spending in 2016-17, rising to £5.9 billion a year in 2019-20.
Past experience indicated that in such circumstances the Army was likely to suffer the most - potentially losing 20,000 of the estimated 30,000 military personnel who would lose their jobs.
Should the Royal Navy be protected due to the need to crew new aircraft carriers, the Army could find itself taking 80% of the total reduction.
If, as the Conservatives have promised, the defence equipment budget is protected, the reductions in personnel would be even greater, rising to 42,000 for the forces as a whole and leaving an Army of just 50,000.
A Government spokesman said they were "committed to spending 2% of GDP on defence."
"Over the next decade we are committed to spending £163 billion on equipment and equipment support to keep Britain safe," the spokesman said.
"The Prime Minister has said that he does not want to see our regular armed services reduced below the level that they are now and remains committed to growing the Reserves to 35,000."