'An attack could happen at any time': Former anti-terror chief says there should be 'no surprise' if terrorists strike Britain before Christmas as he warns 20,000 extremists currently roam our streets

• former anti-terror chief Chris Phillips said an attack before Christmas is likely
• He claimed there were 20,000 names on a terror watch list in the UK
• It comes as ISIS propaganda has threatened attacks on Christmas shoppers 

A terrorist attack in Britain over Christmas is likelier than ever as around 20,000 extremists roam the country's street, a former anti-terror chief has warned.

Chris Phillips, who was once head of the National Counter Terrorism Security Office, has said there should be 'no surprise' if terrorists strike before December 25.

His warning comes after ISIS propaganda threatening to attack Christmas shoppers on Britain's High Streets.

Armed police walk among shoppers at Edinburgh Christmas market as Britain's terror alert remains at 'severe' going during the festive period

'The threat level is severe, which means an attack could happen at any time,' Mr Phillips told the Daily Star Online. 'Attacks are more frequent so no one should be surprised if there is another attack before Christmas.'

He warned that it would be impossible for police to ensure the entire country's safety because of the sheer number of potential threats.

There are 20,000 people on a list. 

Police can't watch them all. Police have to make difficult decisions.'

The UK was hit by four major terror attacks this year at Westminster, Manchester Arena, London Bridge and Parson's Green Tube station. 

The country's current threat level has seen more armed police on the streets and bollards erected on pavements to prevent vehicle attacks. 

ISIS appear to be revving up their propaganda machine ahead of the holidays, producing a series of graphic posters threatening attacks on Christmas shoppers in Europe and the United States.

Last month posters of Santa Claus kneeling before an ISIS executioner on London's Regent Street, and a jihadi with a bloody knife looking out over a Paris Christmas market with the Eiffel Tower in the background were shared online.  

Both images carried the message 'Soon on your holidays' in English, French, and German.