STOCKHOLM, May 20 (Reuters) - Scandinavian airline SAS has increased its flight cancellations to 110 from an earlier total of 25 as it takes precautions in case it fails to avert a pilots' strike set to begin at midnight over proposed changes to staff contracts.

SAS is in talks with the Swedish pilots' association SPF and Norwegian pilots union NSF, whose members oppose a proposed collective agreement that will increase the airline's ability to adapt staffing levels to seasonal swings in demand.

"Our ambition during the night is to continue the constructive negotiations and reach a joint solution," the company said, adding it had cancelled the flights in order to avoid travellers and its own staff becoming stranded.

The cancellations will take effect tonight and on May 21 and a company spokeswoman said they represented 10 percent of Thursday's total flights.

The flights were mainly Swedish domestic flights and international flights from Sweden, she said.

SAS, 50 percent owned by Sweden, Denmark and Norway, is struggling to compete with lower-cost rivals.

It has already struck deals with one of the two Norwegian pilots' unions and the Danish union, together representing more than half of the company's pilots.

A spokeswoman for the Swedish pilots' union said the talks were in an intense phase but it hoped to reach a deal before the deadline at midnight (2200 GMT).

Negotiations with the remaining Norwegian union had the same deadline.

In March, Norwegian Air pilots called off an 11-day strike after reaching an agreement with the budget carrier. (Reporting by Helena Soderpalm, Daniel Dickson and Anna Ringstrom; Editing by Niklas Pollard and David Holmes)