By Daniel Sandford BBC home affairs correspondent

For the next 80 years it operated in the shadows, giving secret advice to ministers on German spies, then Soviet spies, and in more recent years on terrorists and organised criminals.

The public knew it existed. There were successes and scandals.

But what is now officially known as the Security Service was almost completely cut off from the public.

The web site is aimed at people in charge of others' safety

Even the locations of its buildings in Gower Street and Curzon Street were a secret.

Now the Security Service - still popularly known as MI5 - has a grand building on the River Thames just down the road from the Houses of Parliament.

It has had its own website since 1998.

But for the first time on the new updated site, it is giving a public assessment of the threats the UK faces, and open advice to businesses about how to counteract those threats.

Browsing across the site is fascinating for all spy enthusiasts.

It does not give much away, but somehow when you read something on a site called www.mi5.gov.uk it has an authentic ring about it, and it can be chilling.

Threat

This is the assessment of the threat from international terrorism.

"The threat remains real and serious. Al-Qaeda cells and supporters of affiliated groups are known to be active in the UK."

The section on methods of attack says: "It is possible that al-Qaeda may seek to use chemical, biological or radiological material against the West.

"Osama bin Laden has alluded to such devices on several occasions".

But the main purpose of the site is the expert practical advice from MI5's National Security Advice Centre.

It's aimed at people in business and public life who have responsibility for other people's safety.

It gives advice on how to protect people from bombs, and how to vet staff properly before employing them.

It warns of the risks of using contractors who can be easily infiltrated by terrorists.

Openness

It suggests businesses and other organisations contact the Counter Terrorism Security Advisors who are part of every police force for further advice.

The greater openness about the threat and the advice has to a certain extent been forced on MI5 by the business community.

The Unicorn Project was set up last year to engage London firms in counter-terrorism.

It asked companies what they thought about the advice they were getting.

"Incoherent", "outdated" and "condescending" were three of the more polite words that were used.