ALTHOUGH government ministers and security chiefs are not issuing any details of what the raising of the terrorist threat level means in practice, it is an attempt to put the public on notice that the blowback from the Syria crisis could soon impact upon the streets of Britain.
By raising the nationwide security level to “severe” – the second highest of five threat levels – the government is making a very stark statement.
It is worried that an “attack is highly likely” but has no specific intelligence on a particular plot or action by a terrorist group.
By putting its concerns out, the government hopes the members of the public will become extra vigilant and be more likely to report suspicious activity.
In any counter-terrorist strategy, vigilance by the public plays an important part in providing intelligence of terrorist activity.
The intelligence services have excellent eavesdropping technology to monitor e-mail, social media and mobile telephone conversations of suspected terrorists but often tip-offs from the public can provide the key piece of information when building intelligence jigsaws.
The ability of jihadi terrorists groups to pull off spectacular attacks goes all the way back to the 9/11 attacks in New York and Washington in 2001, so the government is rightly concerned that gaps in its intelligence could be exploited by people wishing to do harm to Britain.
While alerting the public, raising the security alert level is also believed to be triggering a series of discrete security measures.
These including the ramping up of police patrols around central London and other sensitive locations such as airports, royal palaces, the homes of public figures and key infrastructure.
Police and security service overtime is also expected to be stepped up as a means of making more personnel available for the additional security activity.
It is probably no coincidence, then, that the raising of the security alert level is taking place less that a week before US president Barack Obama and other Nato leaders are to fly into Britain for the Western alliance summit in Wales.
Some 9,000 police officers are to protect the gathering, with troops and security operatives in support.
With so many high-profile figures concentrated in the famous Celtic Manor hotel in Newport, South Wales, it is not surprising that security chiefs are not taking any chances.
The ghost of 7 July 2005, when terrorist struck London just as Tony Blair was hosting world leaders at Gleneagles, looms large.
• Tim Ripley is a defence analyst and commentator.