Convicted murderers and drug dealers who have recently left prison in Russia face being conscripted to fight in Ukraine under a change to the law.
President Vladimir Putin amended the legislation on calling up reservists to include men convicted of serious crimes who recently left prison.
Former prisoners convicted of sex crimes against children or terrorism are still excluded from serving.
Russian soldiers have been accused of crimes during the invasion of Ukraine.
The Independent International Commission of Inquiry on Ukraine, set up by the UN, reported in September that war crimes had been committed by Russian forces including summary executions of civilians and acts of “sexual gender-based violence” by “some” soldiers.
Ukraine itself says it has identified tens of thousands of possible war crimes by Russian forces.
Russia denies deliberately attacking civilians and accuses Ukrainian forces of targeting civilians in separatist-held areas of the country with artillery, which Ukraine denies.
The UN commission said it had found “two instances of ill-treatment of Russian Federation soldiers by Ukrainian soldiers” but the number of war crimes allegations against Russia was “obviously significantly larger”.
In September, reports emerged that the Wagner mercenary group was recruiting prisoners to fight in Ukraine in exchange for their sentences being commuted.
Russian law does not allow commutation of prison sentences in exchange for mercenary service but Wagner head Yevgeny Prigozhin was filmed telling prisoners “nobody goes back behind bars” if they served with his group.
On Friday, Wagner opened its first official headquarters in Russia, in the city of St Petersburg.
President Putin announced that some 49,000 of about 300,000 reservists called up since September had already been deployed to units serving in Ukraine.
He told a group of young men and women from a Kremlin-controlled movement called Popular Front that “about 50,000” volunteers had also signed up.
Military experts in the West and Ukraine say Mr Putin’s decision to call up reservists showed that Russian troops were failing badly on the battlefield in Ukraine.
Thousands of Russian men opposed to the war have fled the country since the call-up was announced.
Since Russia invaded Ukraine on 24 February, thousands of civilians and combatants have been killed or injured, cities and towns have been destroyed in fighting, and nearly 7.8 million Ukrainians have been registered as refugees in Europe, with 2.8 million of them in Russia.
