Britain's Army could shrink to its smallest size in more than 250 years the UK has been warned.
A report published by the think tank Royal United Services Institute predicts the UK defence budget will fall to 1.95% of GDP – below the NATO minimum of 2%.
It warned that up to 30,000 service personnel could go – with the Army likely to bear the heaviest cuts – leaving the armed forces with a combined strength of just 115,000 by the end of the decade.
In a worst case scenario the Ministry of Defence (MoD) might face a 10% cut over the next four years and would need an additional £3bn added to its budget after 2016 to remain at the same level of spending.
Further increases would then be needed to keep pace with the growth of the economy.
The report's author Professor Malcolm Chalmers states: “By 2019/20 an extension of the commitment to the NATO target would require the MoD to be provided with an additional £5.9bn in annual spending, compared with current plans.”
None of the major parties has guaranteed defence spending and the Foreign Secretary Philip Hammond wouldn't commit on the issue when pressed in an interview at the weekend. UKIP is the only party to suggest it will protect defence.
The US Army Chief of Staff recently warned Britain against making further cuts and Barack Obama has also spoken about the matter to David Cameron in private.
On Thursday MPs will debate defence spending in the House of Commons. It is becoming an increasingly important issue amongst backbench Conservative MPs who don’t understand why the International Development budget is ring-fenced.
Under the last security review in 2010 the Army reduced in size from 102,000 personnel to 82,000; the smallest since the Napoleonic wars.
Major equipment was also scrapped and not replaced, including the country's aircraft carriers and maritime patrol aircraft.
Those decisions have been exposed in recent months with the threat from Islamic State and growing aggression from Russia.
The paper's optimistic scenario envisages defence receiving an extra £4bn per annum from 2019/20, but recognises that would result in severe cuts for other departments or increased taxation.
The report states: “Even on the optimistic scenario, numbers of service personnel could fall from 145,000 to 130,000 by the end of the decade. Under the pessimistic scenario, they could fall to 115,000.”
It also warns than ships, fighter jets and armoured vehicles could be affected.
The report concludes: “In either scenario, the result will be a remarkably sharp reduction in the footprint of defence in UK society over a decade.
“Even in the optimistic scenario, defence's share of GDP will have fallen by a third, from 2.6% of GDP in 2010 to around 1.75% by 2019; and the MoD workforce (service and civilian) will have fallen by around 30% , from 265,740 to 184,000 by 2019.”