Cartoonist Luz has announced he is leaving Charlie Hebdo because working without his murdered colleagues is "too much to bear".

Renald Luzier, known by his pen name Luz, drew the front-cover image of Mohammed after the massacre of the satirical magazine's editorial team in Paris in January.

After more than 20 years at Hebdo, he says he is leaving in September and that his decision has nothing to do with tensions at the paper.

"This is a very personal choice," Luz told French newspaper Liberation. "Each issue is torture because the others are gone.

"Spending sleepless nights summoning the dead, wondering what Charb, Cabu, Honore, Tignous would have done is exhausting," he said, referring to his colleagues killed on 7 January.

He said he had thought about leaving a long time ago, but he "continued in solidarity, to let nobody down".

"Except that at one point, it was too much to bear," he added.

Two Islamist brothers slaughtered 12 people at Hebdo's offices over its cartoons of the Prophet Mohammed.

A week after the massacre, Luz penned the magazine's cover portraying Mohammed with a sign saying "Je Suis Charlie" under the words "All Is Forgiven".

In response, protests, some of which turned deadly, erupted around the world.

Luz, who started working at Hebdo in 1992, has since said he will not draw the prophet again because it no longer interests him.

His resignation will be a blow to Charlie Hebdo, which had been on the verge of bankruptcy before the massacre.

After the attacks, donations poured in from around the world.

But the magazine has become split over the use of the funds, with some staff accusing management of not being transparent enough.

In April, 15 of its 20 staff members, including Luz, called for all employees to become equal shareholders in the magazine.

Hebdo's management has said the donations would be "handed over in full to the victims".