The Nobel Prize-winning head of a Japanese institute whose scientists' work on stem cells was hailed as a game-changer in the field of medical biology called the lead researcher's handling of the data "extremely sloppy" and "irresponsible".

Two papers published in the journal Nature in January detailed a simple way to reprogram mature animal cells back into an embryonic-like state that allows them to generate many types of tissue, offering hope for a simpler way to replace damaged cells or grow new organs in humans.

But other scientists have been unable to replicate the research's results since then and there have been indications of problems with its data and images.

"The problem here is one immature researcher collected a huge amount of research data, and her handling of data was extremely sloppy and irresponsible," president of Japanese research institute RIKEN Ryoji Noyori told a news conference.

"I would like to offer my apology for the Nature articles, having brought into question the credibility of the science community," said Noyori, bowing deeply.

Noyori, who won a Nobel prize for chemistry in 2001, was referring to Haruko Obokata, 30, a lead author of the papers who became an instant celebrity in Japan after they were published.

A written statement from Obokata and two other RIKEN researchers made available at the news conference said they are discussing the possible withdrawal of the papers with other co-authors.

Another scientist on the team, Teruhiko Wakayama of the University of Yamanashi, has already called for the papers to be withdrawn.

"It is no longer clear what's right," Wakayama told public broadcaster NHK on Monday.

The news conference was called to release the interim findings of investigation on the controversy by a panel of experts from within and outside RIKEN.

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Nobel laureate calls handing of stem cell research data 'sloppy'