Wikipedia, one of the world's biggest websites, is losing many of its English-language editors, crippling its ability to keep pace with its mission as a source of knowledge online, a study says.

The study led by Aaron Halfaker of the University of Minnesota found that the number of "collaborators" or volunteer editors has been on the decline from around 56,000 in 2007 to some 35,000 at the end of 2012.

The researchers said there are a number of reasons, including the rise of automated programs or "bots," but also noted that some potential contributors are being discouraged by Wikipedia's structure.

"Several changes the Wikipedia community made to manage quality and consistency in the face of a massive growth in participation have ironically crippled the very growth they were designed to manage," the researchers wrote in last week's American Behavioral Scientist.

"Specifically, the restrictiveness of the encyclopedia's primary quality control mechanism and the algorithmic tools used to reject contributions are implicated as key causes of decreased newcomer retention."

They said that while Wikipedia has sought to root out less competent editors, its rules have also discouraged "desirable newcomers" who get discouraged when their contributions get deleted.

Wikipedia has editions in 285 languages. Its founder Jimmy Wales has cited the need to make Wikipedia more open to newcomers, to keep up with the vast amount of information it is trying to process.

But the authors of the study said Wikipedia is being crimped by its rules trying to improve quality.

"Wikipedia has changed from 'the encyclopedia that anyone can edit' to 'the encyclopedia that anyone who understands the norms, socializes him or herself, dodges the impersonal wall of semi-automated rejection and still wants to voluntarily contribute his or her time and energy can edit,'" they wrote.