Twitter, which currently has a 140-character limit, may launch the service toward the end of the first quarter but has not set an official date, Re/code reported, citing sources familiar with the plans.
The sources said the character limit could change before the final version of the product is unveiled.
The micro-blogging website, in an attempt to keep the current look for the Twitter timeline, is testing a version of the product which displays 140-character long tweets but expands to reveal more content when users click on the tweet, Re/code said.
Twitter has been experimenting under Jack Dorsey - the company's co-founder who returned as chief executive in October - to make the service more engaging.
In the few months under Dorsey, Twitter introduced the 'Moments' feature, added polls to tweets, rolled out a "buy" button and replaced its star-shaped "favourite" icon with a heart-shaped icon called "like".
Twitter has come under increasing pressure to boost user growth and ad revenue. It faced its slowest user growth in 2015 - it now boasts just over 300 million users - and was eclipsed by photo-sharing app Instagram, owned by Facebook Inc, which surpassed 400 million users last year.
However, some users took to Twitter to express their opposition to the new feature with the hashtag #beyond140.
"Just say no to #beyond140!," Andrew Wright tweeted.
Re/code reported in September that the new feature was in the works.
Twitter declined to comment.