The Government has been accused of "slashing" the Army after it confirmed the regular force would be cut to its smallest size since the Boer War.
Defence Secretary Liam Fox said the Government would be investing £1.5 billion in the reserves over the next 10 years, building up the Territorial Army (TA) as the regular force declines.
In a Commons statement, he told MPs that he ultimately envisaged a total force of around 120,000 with a ratio of around 70% (84,000) full-time regulars to 30% (36,000) part-time TA.
That compares with a current regular Army of more than 100,000 with around 14,000 reservists.
Dr Fox said that the changing balance would bring the UK more into line with comparable countries such as the United States, Canada and Australia.
The Vice Chief of the Defence Staff, General Sir Nicholas Houghton, who led a review of the reserves, said the changes would mean the armed forces would be better configured to meet the challenges of the future.
"This is a once in a generation opportunity to change our reserve forces model for the better, to ensure the systemic decline of our reserves is reversed and to enable our armed forces - regulars and reserves - to better meet the security challenges of the future," he said.
For Labour, shadow defence secretary Jim Murphy accused Dr Fox of going back on promises of "thousands of extra troops" made by the Tories when they were in opposition.
"The Army has been slashed to cover up the funding gaps left by the rushed defence review," he said.
"It is hard to conclude anything other than that this is strategic shrinkage by stealth, because today's cut in the Army is bigger than the entire current deployment of all UK forces in Afghanistan."