A think tank has warned that the UK's defence budget could fall to 1.95% of GDP, putting 30,000 service jobs at risk.

The Royal United Services Institute (Rusi) forecasts a fall from the 2.2% expenditure, as of 2013, down to below 2% - the Nato minimum - by the end of the decade.

If it is correct, it could mean that 30,000 service personnel would lose their jobs over the next four years – leaving the armed forces with a combined troupe of 115,000 people. If defence is given as much as health and schools are being promised, then 15,000 jobs will still have to go, the report claims.

The think tank says: "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."

In the optimistic scenario, Rusi says that the defence budget would be given an additional £4bn per year from 2019/20.

However, "In the context of wider austerity in public spending, such an increase is not plausible. The government is not yet convinced that strategic security risks are high enough to justify an exemption for defence from austerity," the Rusi paper claims.

The most pessimistic scenario would see the Ministry of Defence's (MoD) budget slashed by a tenth.

The report adds: "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."