STOCKHOLM, March 1 (Reuters) - Swedish payments firm Klarna nearly tripled its valuation to $31 billion in less than six months after it announced on Monday a new $1 billion fundraising round. The new round, which will make Klarna the most valuable European startup, was oversubscribed four times and included a combination of new and existing investors. The “buy now, pay later” firm completed a $650 million funding round in September from a group of investors led by Silver Lake that valued it at $11 billion. Klarna also said it would pledge 1% of the capital raised to a newly created initiative that focuses on key sustainability challenges around the world and would launch on April 22 on World Earth Day. Reuters had reported last week that the company, which competes with PayPal and Australia’s AfterPay, was finalising another private funding round. (Reporting by Supantha Mukherjee in Stockholm and Abhinav Ramnarayan in London; Editing by Johan Ahlander)