Female rebels were compelled to have abortions so as not to undermine their fighting ability, Attorney General Eduardo Montealegre told reporters.
The left-wing rebels have denied this in the past, saying contraception was readily available.
The Farc has fought a five-decade insurgency in Colombia but peace talks in Cuba have made significant progress.
Agreement has been reached in four broad areas during three years of talks with the Colombian government.
These include how the justice system deals with crimes committed by the rebels and government forces.
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Mr Montealegre said 150 former fighters had testified to being forced to have abortions.
"We have evidence to prove that forced abortion was a policy of the Farc that was based on forcing a female fighter to abort so as not to lose her as an instrument of war," he said.
Government figures suggest there are up to 8,000 fighters, a third of them women.
More than 220,000 people have been killed in the conflict, the majority civilians.
The Farc, which was formed in 1964 with a vow to install a Marxist regime, once controlled a swathe of Colombian territory the size of Switzerland.
But the group has suffered a number of setbacks in recent years and has become increasingly involved in the drug trade.
Colombia is investigating more than 150 cases of forced abortion by Farc rebels, the chief prosecutor says.