Scientists at the top-secret military research unit at Porton Down, Wiltshire, have been assessing the potential use of Ebola as a bioterrorism weapon, according to confidential documents.

A three-page memo, marked ‘UK secret UK eyes only’, reveals that the unit, where chemical, radiological and biological threats are analysed, was tasked with evaluating whether terrorist organisations such as al-Qaida and Islamic State (Isis) could use the deadly virus to attack western targets.

The heavily redacted document, which has been released under the Freedom of Information Act, reveals that the unit was asked last October to provide “guidance on the feasibility and potential impact of a non-state actor exploiting the Ebola outbreak in west Africa for bioterrorism”.

It goes on to explain that non-state actor threat assessments are “provided by the joint terrorism analysis centre”, while threats to “UK deployed forces are provided by defence intelligence”. The memo outlines three possible scenarios under which terrorists might seek to exploit the Ebola outbreak, which so far has killed more than 9,000 people in the three most affected countries, Guinea, Sierra Leone and Liberia.

The first scenario outlined is completely redacted, illustrating the acute sensitivity about the issue. The second scenario is heavily blacked out but, according to the memo, “would be both logistically and technically challenging for a non-state group to undertake”. It observes: “Clearly there are practical issues involved with such a scenario that of themselves are often not insurmountable but taken together add enormously to the complexity of successfully undertaking this attack.”

A third, also heavily redacted, scenario “constitutes the most technically challenging of the scenarios considered here”.

Concerns that terrorist groups might look to “weaponise” Ebola have been raised by several thinktanks and politicians. Last year Francisco Martinez, Spain’s state secretary for security, claimed that Isis fighters were planning to carry out “lone wolf” attacks using biological weapons. Martinez said that his belief was informed by listening in to conversations uncovered in secret chatrooms used by terrorist cells. The claim has since been played down by others.

Jeh Johnson, the US department of homeland security secretary, said last October that “we’ve seen no specific credible intelligence that Isis is attempting to use any sort of disease or virus to attack our homeland”.

Dr Filippa Lentzos, a senior research fellow at King’s College London and an expert on bioterrorism, said terrorists looking to use the virus as a weapon would encounter problems. “It doesn’t spread quickly at all,” she said. “Terrorists are usually after a bang and Ebola isn’t going to give you that.”

On average, a person infected with Ebola will infect two more people. In a developed country such as the UK transmission would be even more limited.

“People with Ebola are infectious only when they show symptoms,” Lentzos said. “Could terrorists go to west Africa, get infected, then come back and sit on the tube? Sure, but they’re not likely to be functional for very long. They’re going to be very sick and you’ll see that. So they would have only a very small window in which to operate. And in a country with a developed public health system like the UK, there would be plenty of chances to clamp down on an outbreak.”

Other biological weapons would potentially be more attractive to terrorists, experts suggested. Unlike Ebola, which requires the transmission of body fluids, anthrax spores can be dried and milled down to form tiny particles that can be inhaled.

However, even the suggestion that Ebola could be weaponised made it a potentially powerful weapon for terrorists, Lentzos suggested. “If your aim is not to kill a lot of people, or even make them ill, but instead to frighten them and cause a huge level of societal disruption, then bioterrorism would do that. It elicits exceptionally high levels of fear, disgust and abhorrence.”

The use of pathogens as a weapon has been tried before. Following the attacks in New York and Washington in 2001, five people died in the US after opening letters laced with anthrax. In the 1980s, a cult in Oregon spread salmonella on salad bars in restaurants in an attempt to keep voters from the polls so its preferred candidates would win.

“The risk of small-scale bioterrorism attacks is possible and very likely,” Lentzos said.

Porton Down is known to have experimented with Ebola but a specific request for the laboratory to analyse the virus’s potential use by “non-state agents” highlights the growing concern that terrorists are becoming increasingly inventive in their choice of weapons.

One scenario could see terrorists combining genes from different pathogens to synthetically create super pathogens that could spread disease far more effectively than Ebola. But Lentzos suggested this was unlikely. “It’s pretty damn hard to make dangerous pathogens from scratch in the lab. Experts have a really hard time doing that. At this point I’m not sure that’s what we need to worry about.”

Lentzos said that focusing on the terrorist threat posed by Ebola risked losing sight of the bigger picture. “To beat Ebola we have to worry less about terrorism and more about public health. Disease knows no borders.”