Budget cuts could see the strength of the armed forces slashed by a fifth over the coming years, a report has warned.
The number of trained military personnel is set to drop from 175,000 to fewer than 142,000 by 2016, according to analysis for the Royal United Services Institute (RUSI).
The dire predictions emerged as defence minister Bill Rammell delivers a keynote speech on the future of the armed forces, including details of spending priorities.
The report, by defence expert Professor Malcolm Chalmers, insisted that efficiency savings "will not be enough" to put Britain's military "on a sustainable footing".
It said that even being "cautiously optimistic", intense pressure on the Government's finances due to the recession meant that the Ministry of Defence budget was likely to fall by 11% in real terms by 2016-17.
A much deeper reduction of some 15% over the next three years "cannot be ruled out", Prof Chalmers added.
The problem will be made worse because the costs of employing troops and civilian personnel have been rising in real terms, as has the cost of procuring and running equipment.
"In addition to the likelihood of significant real reductions in the available budget, defence planners need to take account of continuing growth in the unit costs of defence capabilities.
"The combination of these two trends means that the next six years are likely to see a reduction of around 20% in numbers of service personnel, and a commensurate reduction in numerical military capabilities (major vessels, aircraft and ground formations)."
An MoD spokesman said: "Like all departments, the Ministry of Defence is facing challenging financial circumstances...The Chancellor has already said not a penny will be cut from the defence budget next year but it is not possible to give a meaningful assessment beyond 2010/11 as future spending plans have not yet been set."
