By Tom Gibb
BBC correspondent in Sao Paulo
A law allowing Brazil's air force to shoot down suspected drug planes comes into force on Sunday.
Brazil is a major market for cocaine from Colombia and Bolivia
The government says the measure is necessary to stop the large amount of cocaine coming in both for sale locally and for shipment to other countries.
The shoot-down law has been widely publicised for three months but unregistered flights have continued.
Brazilian Defence Minister Jose Viegas said that all those using light aircraft had been adequately warned.
He said before air force pilots could seek authorisation to shoot down a plane they would have to take eight steps including making visual contact and firing warning shots.
Authorisation can only be given by the head of the air force.
But the authorities have also warned that drug planes that do not obey air force orders will be shot down even if they are carrying children.
Despite the widespread publicity, the number of unregistered flights, averaging more than 12 a day, has not dropped over the last three months.
Most of these are probably farmers who do not register a flight plan so as to avoid taxes.
Brazil produces few drugs but has become a major trans-shipment route for cocaine produced in neighbouring Colombia.
The vast Amazon basin is particularly difficult to police.
Only Colombia has a similar shoot-down policy in South America.
Peru withdrew its law after its air force mistakenly killed a US missionary and her baby daughter after bringing down their plane three years ago.