Brazil is a major market for cocaine from Colombia and Bolivia

President Luiz Inacio Lula da Silva has backed the law, which has waited six years for presidential approval.

His government said aircraft would only be fired at if they failed a series of checks - an attempt to ease fears some planes may be mistakenly targeted.

Brazil is a major transit point and market for trafficked cocaine.

Private aircraft frequently flout the law by transporting the drug over the Amazon rainforest region, which borders the cocaine-growing zones of Colombia and Bolivia.

Brazil says pilots trafficking drugs have been known to make obscene gestures and mock its air force personnel in mid-air, secure in the knowledge that they will not be shot down.

The BBC's Steve Kingstone in Sao Paulo says the new law seeks to cut off the supply line used by smugglers transporting Colombian cocaine destined for Europe or Brazil's large cities.

Peru error

The law was passed by Brazilian Congress in 1998 but is only just coming into effect.

Defence Minister Jose Viegas said the Brazilian air force will have to follow nine steps before attacking suspicious aircraft.

These included efforts to identity suspicious planes, persuading them to land safely and firing warning shots.

Mr Viegas said these checks were enough to answer concerns over the policy voiced by Washington.

The US government had demanded guarantees the law would not lead to a blunder of the kind that resulted in the deaths of five people, including a US citizen and her daughter, in Peru in 2001.

The Peruvian air force shot down their plane after mistakenly believing it was trafficking narcotics.