Tests in the United States indicate a cow may have BSE, or mad cow disease, but more tests are needed to establish whether the animal was really infected.
Mike Johanns was not put off a "good steak" by the new case
The cow, which could not walk - a typical BSE symptom - had twice tested negative to a different type of test.
A ban on US beef was imposed by Japan, once the top US export market, after the first case of BSE was recorded in Washington state in May 2003.
Consumption of beef in the US is still on the rise despite the scare.
A sample will be sent to a special UK-based laboratory for further testing, officials said.
"This animal did not enter the food supply or the feed supply," said Agriculture Secretary Mike Johanns, who played down the alert by saying he was going to enjoy "a good steak".
"It's going to require additional testing before we can confirm one way or another whether this is BSE," said Dr John Clifford, a top veterinary with the Agriculture Department.
It is unclear whether the animal, which was processed at a special facility for animals not suitable for human consumption, was born in the US.
The US was hoping for the Japanese ban to be lifted, but what if confirmed, the new case of BSE might further damage US beef exports.
But Mr Johanns played down the possible consequences.
"I don't anticipate problems with our trading partners," he said.
"They'll want to know what the issues are and what we have done.
"We want to assure them, and assure the public, that what we are doing here is transparent."
So far, the human variant of the degenerative brain disorder, known as Creutzfeldt-Jakob disease, has claimed more than 150 lives - mostly in Britain, which was the epicentre of the outbreak which started in the 1990s.