The Ronald Reagan Presidential Foundation is threatening legal action to prevent a vial of the late US president's blood being sold in an online auction, calling it a "craven act."

The foundation's executive director John Heubusch voiced outrage over the sale of the blood -- dating from the 1981 assassination attempt on Reagan -- by an auction house based on Guernsey, in the British Channel Islands.

"If indeed this story is true, it's a craven act and we will use every legal means to stop its sale or purchase," he said, adding that he had questioned the Washington DC hospital where Reagan was rushed on March 30, 1981.

"We've spoken to GW (George Washington) Hospital and are assured an investigation as to how something like this could possibly happen is underway," he said in a statement.

And he said: "Any individual, including a president of the United States, should feel confident that once they enter into the care of a medical system their privacy and rights are held inviolable."

The five inch long, half-inch diameter phial of blood was being sold by PFC Auctions, accompanied by a laboratory report and a letter from the seller -- who claims he offered it to the Reagan National Library, but was turned down.

"These articles have actually been in my family's possession since 03/30/1981, the day that President Reagan was shot," said the seller, adding that his mother worked for the laboratory which tested the blood.

"The testing was completed and the test tube was sitting on my mother's desk. At the end of the week, she asked the director of her laboratory if she could keep the paper work and the test tube.

"The director of the lab told her no problem and really never gave it a second thought. It has been in my family ever since."

Then, about three or four months ago, the seller said he contacted the Reagan National Library and spoke with its head, a federal agent, who told him he had to make a few calls to check what to do.

"He called back in 25 minutes and said that everything was ok, National Archives was not interested in what I had, nor was the Secret Service, the FBI and other agencies.

"Since 30 years had passed by, he thought that it was simply something that was of no importance at this time and that I was free to do with whatever I wanted with it."

As of late Tuesday, the highest bid for the vial was 7,587 pounds. The auction is due to end on May 24 at 1900 GMT, it said.

[This photo was taken moments before an attempt to kill then-president Ronald Reagan in 1981. AFP Photo/Mike Evens]