In an unexpected move, the European Parliament voted to require all EU importers of four minerals to be certified to ensure they do not fuel conflicts and human rights abuses.
The European Commission had recommended that checks be only voluntary.
Human rights groups praised the vote as a "historic moment".
The decision will now be discussed by member states and the European Commission.
Amnesty International and Global Witness had originally warned that a voluntary system covering European businesses would have watered down the proposals.
Before the vote, the two groups were among more than 150 which signed a letter calling for binding rules that focused on more manufacturers and more materials.
MEPs decided to back a law covering not just European importers that sourced minerals from conflict zones but also companies that used the four materials - tin, tungsten, tantalum and gold - in the manufacturing process.
While an estimated 20 refineries and smelters would be directly affected, the EU also estimates as many as 880,000 firms would have to provide details on the measures they take to avoid the use of minerals from conflict areas such as the Democratic Republic of Congo and the Great Lakes area of East Africa.
Many tech gadgets such as smartphones use conflict materials, so-called because they are mined in warlike conditions and traded by armed groups.
Campaigners had earlier complained that the scheme did not cover minerals and materials such as diamonds, jade and chromite, which are also known to be sources of conflict.
Euro MPs have called for companies to carry out mandatory checks to stop "conflict minerals" entering Europe from war zones.