Prof Chalmers foresees cuts no matter which party is in power
The British armed forces could be forced to shrink by up to a fifth because of a lack of money, a military think tank has predicted.
The Royal United Services Institute said the number of trained military personnel could fall from 175,000 to little more than 140,000 by 2016.
Its report said the cost of troops and equipment was rising, and major cuts were "inevitable".
The Ministry of Defence said budgets would not be cut at all next year.
The report's author, defence expert Professor Malcolm Chalmers, warned hard choices lay ahead and efficiency savings would not be enough to put Britain's defences on a sustainable footing.
He said even being "cautiously optimistic", intense pressure on government finances meant the MoD's budget was likely to fall by 11% in real terms by 2017.
And he said a much deeper reduction of about 15% over the next three years could not be ruled out.
Unless defence is ring-fenced it will face cuts of the sort of magnitude we're talking about
Professor Malcolm Chalmers
Professor Chalmers warned the problem would be made worse because the costs of employing troops and civilian personnel have been rising in real terms, as has buying and running equipment.
Cuts to the available budget combined with growing costs meant the next six years were likely to see a reduction of about 20% in the number of service personnel, the report said.
Military capabilities in terms of ships, aircraft and ground formations would also be reduced by a similar amount.
Professor Chalmers indicated that major cuts would be inevitable whichever party was in power later this year.
He said there would be a strong temptation for a new government to postpone making tough, potentially unpopular choices, perhaps by only looking a few years ahead, rather than a whole decade, when reviewing defence.
He warned ministers would face the choice between suffering the "political pain" of defence cuts all at once, or in "successive small doses".
Professor Chalmers told the BBC: "Defence is something you have to plan on a long-term basis, and it's much better to plan it long-term than year to year because the implications of that is more and more waste and inefficiency.
"Given the fact that we all know there is going to be a severe squeeze in the overall government budget, unless defence is ring-fenced it will face cuts of the sort of magnitude we're talking about."
Former Liberal Democrat leader Sir Menzies Campbell said spending cuts would be painful.
"One thing is certain, there has to be a review because the present arrangements have resulted in very considerable overstretch, procurement has had a pretty shabby history in truth," he said.
"And against the background of the general economic situation there's a very strong argument that says we should be protecting frontline services in health and education, and defence has to take its share of the damage."
The institute's predictions come ahead of defence minister Bill Rammell's keynote speech on the future of the armed forces.
He is expected to focus on "the impact of modern society on defence" such as the Freedom of Information Act and 24-hour news.
Responding to the report, an MoD spokesman said it welcomed the RUSI's contribution to the debate, and said "like all departments, the Ministry of Defence is facing challenging financial circumstances".
"We routinely review spending to ensure we focus on Afghanistan and live within our means," he added.
The MoD said the chancellor had already said not a penny would be cut from the defence budget next year - but it was not possible to give a meaningful assessment beyond 2011 as future spending plans had not been set.
The Conservatives and Labour have said they will hold strategic defence reviews after the general election. The last was held in 1998.