* Routes from Duesseldorf to Caribbean to end from Sept 25
* Other cancellations brought forward
* Sources say lessor asked for planes to be returned (Adds detail on leases, Woehrl bid)
BERLIN, Sept 11 (Reuters) - Insolvent German airline Air Berlin is scrapping more long-haul flights, this time to the Caribbean from Duesseldorf, because it needs to reduce the fleet due to preliminary insolvency proceedings, it said on Monday.
Air Berlin, Germany's second-largest airline, was forced to file for bankruptcy protection last month after shareholder Etihad Airways withdrew funding following years of losses.
Its planes are being kept in the air thanks to a 150 million euro ($180 million) government bridging loan, which will last until the middle of November at the latest.
Industry sources said the routes were being cut after a leasing company asked for the A330 planes that fly the long-haul routes to be returned.
Ireland-based lessor Avolon declined to comment when asked about the status of its planes with Air Berlin. Avolon leases 10 A330s to Air Berlin, another source said.
Routes from Duesseldorf to destinations such as Mexico, Cuba, the Dominican Republic and the Antilles will end from Sept. 25, Air Berlin said. It is also bringing forward the cancellation of other long-haul services, including from Berlin to Los Angeles and San Francisco, and from Duesseldorf to Boston.
Bidders are lining up to buy the airline's assets, with Lufthansa seen in pole position to acquire large parts of its rival.
Bidders have until Sept. 15 to submit binding offers, with a decision possible as early as Sept. 21, three days before a German national election.
A source has told Reuters Lufthansa is interested in as many as 90 of Air Berlin's planes, including 38 aircraft it is already leasing from the airline and its leisure unit Niki.
Thomas Cook's German carrier Condor is preparing a bid for parts of Air Berlin, a source said, while media reports say easyJet may be interested in up to 40 planes.
Aviation investor Hans Rudolf Woehrl late on Sunday said he had submitted a bid for the whole of Air Berlin and invited other interested parties, including Lufthansa, Condor and former motor racing driver Niki Lauda to join him.
However, his offer did not find any immediate takers on Monday. Lufthansa and Lauda declined to comment, while at Condor a source familiar with the matter said the German carrier was concentrating on its own bid.
Smaller investors interested in Air Berlin include German family-owned logistics company Zeitfracht. Others who initially expressed an interest, including Ryanair and German businessman Alexander Skora, have since stepped back from a bid.
($1 = 0.8323 euros) (Reporting by Victoria Bryan and Klaus Lauer; Editing by Mark Potter and Susan Thomas)