All human embryonic stem cell lines approved for use in federally funded research are contaminated with a foreign molecule from mice that may make them risky for use in medical therapies, according to a study released Sunday.

Researchers at the University of California, San Diego, and the Salk Institute for Biological Studies in San Diego report that if the stem cells are transplanted into people, the cells could provoke an immune system attack that would wipe out their ability to deliver cures for diseases such as Parkinson’s, Alzheimer’s and diabetes.

The finding is a setback to the Bush administration’s controversial policy that only provides federal funding for research using embryonic stem cell lines that were created before August 2001. Evidence that all such stem cell lines are contaminated fuels long-standing concern among leading researchers that the lines eligible for federal money are insufficient to propel research forward.

The scientists who wrote the study say it could take at least a year or two, if it is possible at all, to find a way to salvage the stem cells by wiping them clean of the mouse molecules.

“We don’t know, but I’m trying to be optimistic,” said Fred Gage, a professor of genetics at the Salk Institute who co-authored the paper in the current issue of Nature Medicine.

The researchers said the safest course was to create fresh batches of stem cells that were free of contamination from animal molecules — a process that could also take years.

The need to develop new, uncontaminated embryonic stem cell lines would bolster the influence of the California Institute for Regenerative Medicine, a $3 billion funding agency established by state voters in November to circumvent the Bush restrictions.

“This is why Prop. 71 is so important,” Susan Fisher, a UC San Francisco professor of cell and tissue biology, said of California’s stem cell research measure. “We will be able to do this basic research to be able to really produce a strong foundation on which this work can continue.”

When the stem cells were first isolated, they were grown in petri dishes lined with cells from mice and bathed in blood serum from calves and other animals. The animal material was used to encourage the stem cells to multiply while preserving their unusual ability to mature into any kind of human cell.

Researchers have suspected that exposing the stem cells to animal products could have contaminated them with viruses, proteins or other molecules that could be dangerous to people.

Now they have evidence that it did.

According to the study published Sunday, human stem cells have incorporated a type of sialic acid that is common in many mammals but isn’t produced by people.