WASHINGTON, Feb 22 (Reuters) - Stepping into a heated political and ethical debate, 80 U.S. Nobel laureates have signed a letter to President George W. Bush urging him to not block the first flow of federal grants for research on human embryo cells, The Washington Post reported Thursday.  

The letter, to be faxed to the White House on Thursday morning, marks the latest effort to influence the Bush Administration as it decides whether to fund experiments on embryonic stem cells, the Post said.

The letter comes three weeks before a National Institutes of Health deadline by which scientists must apply for the agency's planned first round of stem cell research grants, the report noted.  

Researchers say stem cells, taken from frozen embryos that fertility clinics were planning to discard, can one day cure a range of diseases from diabetes to paralysis.  

Opponents call the research immoral.  

But, according to the Post, the Nobel laureates say in their letter to Bush that given the cells' great therapeutic promise, it would be immoral not to study them.  

The letter was signed by such notables as James Watson, who won a Nobel in 1962 for co-discovering, with Francis Crick, the structure of DNA; molecular biologist Hamilton O. Smith, who was a key player in the recent landmark genome mapping effort by Celera Genomics of Rockville, Maryland; and Edward Lewis, the California Institute of Technology biologist who conducted seminal work on embryo development, according to the Post.  

The letter was composed and circulated by Michael West and Robert Lanza, two scientists at Advanced Cell Technology Inc., a biotechnology company in Worcester, Massachusetts, the Post said.  

Opponents of the research, including Douglas Johnson, legislative director for the National Right to Life Committee, criticized the letter, the Post said.  

"Just as war is too important to be left only to generals, the killing of human beings in medical research is an issue too important to be left only to scientists, even Nobel laureates," Johnson is quoted as saying.  

Health and Human Services Secretary Tommy Thompson has said he is reviewing the Clinton Administration's decision to fund stem cell research, the Post said.