Lufthansa had tried to stop the strike, the 13th in 18 months, with a temporary injunction but on Tuesday night two courts ruled in favour of the pilots' union, Vereinigung Cockpit.
"If there's no new offer from Lufthansa we will call for a further strike," VC spokesman Markus Wahl told Reuters at Frankfurt airport.
He said strikes could occur at any time, though the union would give 24 hours' notice.
The pilots are striking over retirement benefits, pay and Lufthansa's plans to expand low-cost operations. They want Lufthansa to halt the expansion of Eurowings, which has an Austria operating licence, while negotiations continue.
Lufthansa management has said it must achieve cost cuts in order to compete with budget rivals such as Ryanair, which are targeting the German market.
Ryanair on Wednesday raised its full-year profit guidance again, this time by 25 percent, thanks to strong summer traffic.
German employers' association BDA said the strikes were damaging the entire German economy and the aims of the pilots were legally questionable.
"The question of which planes, which carriers and which company units to use on which routes falls under the freedom to make commercial decisions and should not be attacked through industrial action," BDA President Ingo Kramer told a regional German paper.