The security service, MI5, has advised British businesses that the threat from international terrorism in the UK is lower than at any time since the September 11 terror attacks on the US.

In an advisory note to leading businesses in recent days, the terrorist threat has been downgraded from its second highest level “severe general” to a lower category of “substantial”.

Under a system agreed by the security service two years ago, businesses receive written risk assessments and regular briefings on terrorist threats. These are not made public.

Government policy is to share intelligence more broadly only if it receives reliable information of an imminent attack. “The threat from international terrorism to the UK remains real and serious,” the Home Office said on Tuesday night.

Ministers have generally preferred not to follow the US, where the government has issued public terrorism alerts. The latest advice to businesses is based on data gathered by a secretive government intelligence committee, the Joint Terrorism Analysis Centre, based in central London.

However, UK security experts insist more detailed intelligence messages still coming through leave no room for complacency.

Paul Eskrit, a security adviser to the Corporation of London, said alert levels remained high in the City and around certain government buildings up until the general election amid fears the event might spark of a major terrorist attack as occurred in March last year in Spain.

“The downgrading does not offer a great deal of comfort . . . I for one am not going to advise that we relax our security systems,” he said.