The proposed change could see pilots having to work up to 16 hours a day - two hours more than the current maximum.
"We pilots will no longer be able to guarantee the safety of passengers," if the new rules are allowed to come into force without adjustment next year, said President pilots' association Cockpit (VC) Ilja Schulz.
The current rules already mean that "at the last landing, you've been awake at least 16 hours," Schulz said, speaking at the Berlin Air show at Schönefeld airport.
In a recent survey, Cockpit found that 93 percent of its members had flown while over-tired, while 37 percent had admitted to having a nap in the air after consulting with the co-pilot.
The new regulations were recommended by the European Aviation Safety Agency (EASA), though it will only publish a final proposal at the end of September. The European Parliament and European Council will then make the final decision.
Schulz accused the EASA of putting economics above safety, and said that Cockpit would fight the proposal in court if it is put into force.
"Personally I can only recommend every pilot to stop work if they get tired," said Schulz. The law allows pilots to make unscheduled landings if that happens.