The word "tweet" may have entered the international lexicon thanks to an explosion in 140-character microblogging messages, but an attempt by Twitter's founders to trademark it has been rejected.

Twitter applied to the US patent and trademark office last month for ownership of the word but the request was provisionally denied on the grounds that other companies had filed for trademarks of very similar words.

The authority cited three applications by firms called TweetMarks, Cotweet and Tweetphoto, and said there was a "likelihood of confusion" between the various marks.

The ruling is a setback to Twitter's co-founder Biz Stone, who is keen to protect the rapidly growing language surrounding the service.

In a blog last month, Stone told users that Twitter-related applications that included the word tweet, such as Tweetdeck, Tweetboard and Tweetie, were not a problem.

"We have applied to trademark tweet because it is clearly attached to Twitter from a brand perspective but we have no intention of 'going after' the wonderful applications and services that use the word in their name when associated with Twitter," he said.

Since Twitter's launch three years ago, the word tweet has achieved a degree of respectability. To tweet was recently added to the Associated Press's style guide as a verb. Australia's Macquarie dictionary added tweeting to its latest edition.

• This article was amended on Wednesday 26 August 2009. A "tweet" can be up to 140 characters in length, not 140 words as we said. This has been corrected.