Cambodia's national assembly has voted to introduce conscription so young men aged between 18 and 30 will be liable to serve 18 months in the military.
Around half of Cambodia's population is currently under 18
The move comes despite years of international efforts to reduce the military's excessive size.
A government spokesman said conscription was an important way of reinforcing the army.
But it may be a crude attempt to head off a looming unemployment crisis, a BBC correspondent says.
Worsening job situation
Defence Minister Tea Banh told reporters that while Cambodia's army was numerically strong, many of its soldiers were not fit to serve.
"Many of the military personnel are old and physically fragile," he explained.
"We have to train our young people in the arts of the military," he said. "We need our sons to serve our country in case a bad situation comes along."
However, opposition leader Sam Rainsy warned that the new law would help the government hide one of its major failures - unemployment.
"Every year, around 300,000 young people reach the age of 18 and cannot find jobs," he said.
"In order to control these young jobless people, they are forcing them to enrol in the army," he told reporters.
A government spokesman said that a strong military force was necessary so that neighbouring countries would not look down on Cambodia and conscription was an important method of reinforcement.
The job situation is likely to worsen in the near future as around half the population is currently under 18.
Decades of conflict left Cambodia with an excessive number of troops for such a small country, and demobilisation efforts have been under way for more than a decade.
International donors have pumped millions of dollars into projects to reduce the number of people on the armed forces payroll.
Earlier this month it looked like those efforts might finally produce a significant result, says the BBC's Guy Delauney in Phnom Penh.
A visiting Chinese military official told the media that Cambodia had plans to cut more than a third of its troops.
But his Cambodian counterpart downplayed the comments.
Now the national assembly has passed a bill which could increase troop numbers massively for years to come, our correspondent says.