US President Barack Obama has lifted restrictions on federal funding for research on new stem cell lines.
Mr Obama signed an executive order in a major reversal of US policy, pledging to "vigorously support" new research.
Ex-President George W Bush blocked the use of any government money to fund research on human embryonic stem cell lines created after 9 August 2001.
Scientists say the research will lead to medical breakthroughs, but many religious groups are opposed to it.
Announcing the new policy, Mr Obama said he was authorising a change "so many scientists and researchers and doctors and innovators, patients and loved ones have hoped for and fought for these past eight years".
Opinion polls suggest most Americans support stem cell research, reports the BBC's Richard Lister, in Washington, but the National Right to Life Committee described the move as a "slippery slope". It has also been condemned by the Vatican.
"But scientists believe these tiny cells may have the potential to help us understand and possibly cure some of our most devastating diseases and conditions."
He invoked the example of the late Superman actor Christopher Reeve, who became a staunch advocate of stem cell research after being paralysed in a horse-riding accident.
Analysts say Mr Obama's decision could also lead Congress to overturn a ban on spending tax dollars to create embryos.
That ban, known as the Dickey-Wicker amendment, has been in place since 1996 and renewed every year by Congress.
But Democrat Congresswoman Diana DeGette told the New York Times newspaper that several anti-abortion colleagues were open to the possibility of reversing the ban if this was necessary to help research.
The repudiation is of a policy but also of a way of life, that puts faith above science
Before signing the executive order, Mr Obama said he hoped Congress would act on a bipartisan basis "to further support this research".
Stem cells are cells with the capacity to turn into any other type of human cell, be it bone, muscle or nerve cell.
One embryo can provide a limitless supply because the cell lines can be grown indefinitely.
But the use of human embryonic stem cells in research is controversial with some campaigners saying it is unethical.
The practice of creating embryos is routine in private clinics, but the ban put constraints on federal researchers even before the restrictions imposed by former President Bush, forcing them to use embryos left over from fertility treatments.
Correspondents say the policy change is part of President Obama's pledge to make clear that his administration wants scientific research to be free from political interference.
STEM CELL TECHNOLOGY
Key sources for stem cells are adult organs or embryonic cells
Adult stem cells are identified and separated from other cells
Embryonic stem cells are removed from 5-day-old embryos
Cells are manipulated to stimulate them to take on a specific function
Specialised cells may then be used to treat unhealthy areas
Announcing his development, he described himself as a man of faith who had carefully weighed the implications of the decision, and said moving forward required a "delicate balance".
Like Mr Bush, President Obama has profound Christian beliefs but he has defined the issue in terms of integrity.
To that end, he also signed a memorandum directing the White House's science and technology office to develop a strategy for restoring scientific integrity to government.
And he vowed that only research meeting strict ethical guidelines would be allowed, stressing that under no circumstances would stem cells be used for research into human cloning.
"It is dangerous, profoundly wrong, and has no place in our society, or any society," Mr Obama said.
The move to lift the ban on federal funding was welcomed by stem cell researchers but criticised by opponents and social conservatives.
Peter Wilderotter, president of the Christopher and Dana Foundation, praised Mr Obama for "removing politics from science" and freeing researchers.
President Bush and other social conservatives argued that the embryos are human life and therefore should not be destroyed. Mr Bush twice vetoed congressional attempts to have the ban lifted.
Following Mr Obama's announcement, John Boehner, the Republican leader in the House of Representatives, said the president had undermined "protections for innocent life, further dividing our nation at a time when we need greater unity to tackle the challenges before us".
MEDIA REACTION TO PRESIDENT OBAMA'S PLAN
The dispute is not about whether stem cell science should proceed; it is about how it will proceed.
The stem cell signing - like the economic stimulus bill - is an example of the two different tracks that Obama and Republicans are currently on. Obama, with his base solidly on his side, is making policy with broad support among independents. Republicans, on the other hand, remain in the wilderness and are looking to rebuild from core principles.
Although President Bush limited federal funding of embryonic stem cell research to a few existing stem cell colonies, he did not make such research illegal... Nothing notable has occurred in embryonic stem cell research other than the scientific fraud committed by the infamous South Korean researcher Hwang Woo-suk.
There is real hope that stem cells will soon be available to regenerate tissue for any part of the body. Throughout the past eight years, funding restrictions have forced researchers to develop workarounds... President Obama's executive order presents an excellent opportunity to renew America's commitment to this whole promising field.
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