TERROR THREAT: SAS are plotting to stop attacks happening next weekend

The threat, just days after the attacks in Spain, was revealed when high-level intelligence was intercepted about “extremist activity” at major public events.

Millions of people will fill the streets for the Notting Hill Carnival and police hope to use a face recognition system to spot any suspects.

Police are also considering placing concrete and metal barriers around the carnival area for the first time to protect revellers.

Manchester, Birmingham and Glasgow all have big events planned and airports will face a challenge as millions return from summer breaks.

Yesterday, heavily armed police were out in force at V Festival sites in Weston Park, Staffs, and Chelmsford, Essex.

More than 600 Islamic State fighters are estimated to have fl ed from Syria to Europe since April according to the security services with some returning to the UK.

NOTTING HILL CARNIVAL: Millions of people will fill the streets for the annual carnival

Many of these hardened extremists fought Coalition troops in Raqqa and have pledged to strike terror across Europe.

Known extremists are tracked in the UK by the Joint Terrorist Analysis Centre (JTAC) and are regularly detained on return, but so called “clean skins” arrive via Calais and other ports.

Counter-terror police operate undercover teams of Muslim officers across the UK, tracking the phone calls of extremist suspects – particularly in areas where hard-line groups are known to operate.

Sources say several extremists have been “observed” for weeks and mobile communications chatter has highlighted their movements next weekend.

SAS troops were deployed in June when terrorists attacked crowds at London Bridge.

They arrived within minutes aboard two unmarked Dauphin helicopters, dubbed Blue Thunder, which landed on the bridge.

Army units, from bomb disposal to medics, are on permanent high alert to provide “civil support” as part of a plan called Operation Temperer.

Military chiefs have revealed that a specialist unit ready to support the Counter Terror Command has been undergoing special terror training.

The soldiers, from the Special Forces Support Group, have been practising live firing training using ordinary cars as part of their readiness role.

According to their regimental magazine the unit forms part of the UK counter-terror force and has been using civilian vehicles to prepare for any possible action.

A senior source said last night: “Intelligence in stopping attacks is paramount and it is the public who often report something that makes a major difference.”