The warning came after the German flag carrier said it had concluded a short-term agreement with the pilots' Cockpit union (VC) to cut wage costs.
The deal means the airline will hold back from implementing redundancies until the second quarter of 2021.
However, “the significant overcapacity of pilots will last considerably beyond March 2021,” Lufthansa said.
“The number of redundancies for operational reasons can therefore only be limited by concluding a long-term crisis agreement,” the company added.
The deal with VC cuts short-time work compensation and pension benefits from September to the end of the year, and wage increases planned for 2020 will be postponed until January 2021, the airline said.
“We clearly reject Lufthansa's threat to make compulsory redundancies,” Markus Wahl, VC President, said.
“Pilots are prepared to make a tangible contribution to keeping all cockpit personnel on board.”
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The measures apply to the pilots of Lufthansa, Lufthansa Cargo, Lufthansa Aviation Training and Germanwings pilots, according to the company.
Lufthansa, which also owns Swiss, Brussels and Austria Airlines, said it wouldn't hire pilots from outside the group as long as there is an overcapacity of cockpit staff.
The carrier said earlier this month it would have to make forced redundancies and cut its fleet by at least 100 aircraft after what its chief executive called “the worst quarterly results in Lufthansa's 65-year history.”
The group, which received a government bailout worth €9 billion ($10 billion) in June, has estimated that 22,000 jobs will have to go as demand for flights is expected to stay below pre-pandemic levels for years.
Lufthansa last week walked out of talks with union representatives of German ground staff, saying without union backing for its cost-saving proposals, forced job cuts “can no longer be avoided.”