The official level of terrorist threats to the UK has been made public on the websites of MI5 and the Home Office.
The threat level was reduced prior to the 7 July attacks
The launch of a new system follows criticism by a parliamentary committee of alert levels before last year's 7 July bombings in London.
Levels of alert have been reduced from seven levels to five - low, moderate, substantial, severe and critical.
On Tuesday morning the MI5 website said the threat was "severe", which meant an attack was "highly likely".
The new system combines the threats "severe general" and "severe defined" into a single "severe" category, and gets rid of the previous lowest status "negligible".
The threat level under the old system has been "severe general" since August last year.
Home Secretary John Reid confirmed plans for a shake-up last month after a report by the cross-party Intelligence and Security Committee (ISC) called for a more transparent warning system.
The committee highlighted that the threat level had been reduced from "severe general" to "substantial" prior to the London attacks, and said the old system had provided "inappropriate reassurance" to the public.
The new UK scheme closely resembles the one in the US, launched in March 2002, which has five colour-coded levels.
In Australia there is a four-grade system which was introduced in June 2003.
Shadow home secretary David Davis said: "We welcome this new public system of alert and threat levels. We have been calling for this for some time."
A new government website is also being set up to display the information at www.intelligence.gov.uk.