Some 30% of all farm land is to go to South Africa's black majority by 2014
South Africa says it will take over any land allocated to black farmers which is not being used effectively under a land redistribution programme.
The measure, which takes immediate effect, was announced by Agriculture Minister Lulu Xingwana, who warned farmers should "use it or lose it".
She said confiscated land would go to emerging farmers and co-operatives.
After apartheid, the government set a target to give 30% of all agricultural land to the black majority by 2014.
About 95% of that land has been handed over but much of it has lain idle for years.
The government programme included restitution, by which ancestral land was returned to black communities from whom it was taken before apartheid ended in 1994.
It also involved redistribution, under which black farmers were allowed to secure loans to buy land from the government.
The agriculture minister said at a news briefing on Wednesday: "I have... instructed my directors general to, with immediate effect, implement the principle of use it or lose it.
"Therefore, those who do not use the land must immediately be removed and the land must be given to emerging farmers and co-operatives."
Land reform is a sensitive issue in South Africa and has been brought into sharp focus by the decline of agriculture in neighbouring Zimbabwe, where many white commercial farmers have been violently evicted.