The security service MI5 has published its terrorist threat assessment and safety advice for the first time.
David Blunkett visited the MI5 offices for a briefing on the new site
The details, available to the public on a new website, were previously given only to a few organisations.
But MI5 director general Eliza Manningham-Buller said it was important to help more people - especially businesses - protect themselves.
The current assessment is that "the threat from international terrorism remains real and serious".
The warning comes as the US released figures which suggest terrorist attacks are at an international 30-year low.
MI5 says the main terrorist danger to the UK and to British interests overseas comes from al-Qaeda and associated groups.
"Osama bin Laden has in several statements publicly named Britain and British interests as a target, and encouraged attacks to be carried out against them," it says.
Al-Qaeda cells and supporters of affiliated groups are known to be active in the UK, MI5 confirms on the site.
It also publishes a top 10 list of safety tips for businesses and other organisations.
Bomb blast net curtains
These include advice to carry out risk assessments, look at mail-handling procedures, and check that staff are who they say they are.
Another section advises organisations on protection against flying glass.
Experts recommend applying transparent polyester anti-shatter film (ASF) to glass, to reduce fragments and splinters.
TEN TIPS FOR BUSINESSES
1. Carry out a risk assessment and seek police advice
2. For new premises plan security from the outset
3. Make security awareness part of the culture
4. Keep gardens free from dense shrubbery
5. Make sure access points are kept to a minimum
6. Locks on windows and doors, CCTV, alarms, lighting
7. Set up mailroom away from main premises and train staff
8. Ensure new recruits are who they say they are
9. Use reputable IT people to help protect your information
10. Plan how you will function if something happens
Timber-framed Georgian-style windows should also have bomb blast net curtains, says MI5.
For new buildings blast resistant laminated glass or secondary glazing should be included in the design.
The new site also lists the methods of attack most likely to be used by international terrorists, with bombings most common for al-Qaeda.
Shootings, abductions and kidnappings have also been used and although no such attacks have yet been unleashed on the UK "al-Qaeda may seek to use chemical, biological or radiological material against the West," said MI5.
Businesses are urged to protect information as terrorists are likely to try to get access to details that would be useful to them, by infiltrating organisations or getting help from an "insider".
Two sections of the website have been translated into Arabic to "build on the co-operation of the Muslim community" said the security service.
Additional languages will be added later.
Ms Manningham-Buller said MI5 wanted to share some of its information about the threats.
"For the most part details of our operations must and should remain secret," she said in a statement published on the website.
"But stopping terrorists is only one part of our collective defences against terrorism.
"Another part of our work is to use the knowledge we have about these organisations to provide sensible and practical advice on how best to protect yourself against these threats."
Dr James Hart, commissioner of police for the City of London, said the website would be "an enormous advantage" to the counter-terrorism effort.
Conservative homeland security spokesman Patrick Mercer welcomed the website, but said it was long overdue.
MI5 also lists Northern-Ireland related terrorism, espionage and the proliferation of weapons of mass destruction as continuing threats to the UK.