The lab - which is not being named - is run by UK personnel, not the US military, said Colonel Steve Warren.
"Another lot came up positive and that lot had sent samples to the UK and Massachusetts," the colonel said, referring to the deadly bacteria.
It brings to 68 the number of labs involved in the anthrax scare from the US, including four foreign countries.
A number of US military facilities for the past decade have shipped inactivated anthrax samples to labs around the world to help their research into biological warfare.
But in recent weeks, it has come to light that some suspect samples were not properly de-activated before being posted.
After Tuesday's briefing, Col Warren told the BBC they could not be sure the UK received the live samples but it was sent samples from a larger batch - Lot 5 - in which some of the bacteria was found.
Staff members at some of the labs have been treated for anthrax exposure as a precaution, but no-one has fallen ill.
In addition to labs around the US, suspect samples were sent to facilities in Australia, Canada, South Korea and the UK.
Experts in bio-safety have heavily criticised the lapse and called for improved precautions.
Symptoms of anthrax exposure include skin ulcers, nausea, vomiting and fever, and can cause death if untreated.
The military has ordered all of its labs that have previously received inactive anthrax samples to test them. In addition it is advising all labs to cease working with these samples until told otherwise.
The Pentagon has maintained there is no risk to the general public.
What is anthrax?
Anthrax bacteria live primarily in inactive spores and are found naturally in the soil, but people can ingest or inhale spores, which can make the anthrax active.
Not all will become ill when exposed to anthrax but left untreated, anthrax illness can turn very serious or lead to death.
A blood culture test is used to confirm anthrax exposure.
Anthrax entered the US national consciousness in 2001, when shortly after the 9/11 attacks, letters containing powdered anthrax arrived at news organisations and the offices of US senators. Twenty-two people were affected and of those, five people died.
Sources: FBI Anthrax investigation; CDC, San Francisco Department of Public Health

Galit Nakash is to take a DNA test to determine if the girl is her biological daughter, following a court hearing.
If the results are negative, authorities say they will return the child to Sri Lanka straight away.
Ms Nakash was stopped when airport officials found she had used another child's passport to leave Sri Lanka.
She says she gave birth at a hospital in Sri Lanka during a business trip but that her visa application for the baby, named Tahel, was turned down.
On Tuesday a court in Tel Aviv accepted her request to undergo a DNA test in Israel.
Ms Nakash's lawyer, Matan Hodorov, said his client and the one-year-old child are being moved to a facility at Ayalon prison in Ramle until the results of the test are known on Friday.
Mr Hodorov said Ms Nakash, 49, turned down an Israeli request to take a DNA test while she was in Sri Lanka because she did not trust the process there.
He said when Ms Nakash, who runs an export business and comes from Tel Aviv, re-entered Israel, the passport she used for Tahel carried the name of a different child.
However he said he was "confident the DNA test will prove Galit is the mother [of Tahel].
"We have all the correct medical documents from the hospital in Sri Lanka where she was born," he told the BBC.
The Israeli foreign ministry said Ms Nakash was denied a visa for Tahel because officials had doubts about her story.
"There were a few indications which raised a certain amount of suspicion," spokesman Emmanuel Nahshon said. "That is why we did not allow the child and Galit to enter the State of Israel.
"It doesn't mean she is wrong but we have a legal duty to check it out. If the DNA tests show she's the mother then that's wonderful and the problem is over. If not, the child will immediately be sent back to where she came from, as required under the Hague Convention [on child abduction]," Mr Nahshon said.
"Nobody is trying to take the child away or create a tragedy, but the circumstances are extremely suspicious."
A lab in the UK may have received a sample of live anthrax from the Pentagon in 2007, say US officials.
[NEXT_CONCEPT]
An Israeli woman and a baby who returned from Sri Lanka have been kept at Tel Aviv airport since Saturday amid questions over the child's origins.