Around 200 undercover SAS soldiers will be based in secret locations in London, Birmingham, Leeds and Manchester in order to combat any possible terrorist threat.
The plans for mobilisation came after the Home Office responded to a warning from MI5’s Joint Terrorism Analysis Centre that ISIS extremists were targeting mainland Britain for a major terror attack.
The 200-strong force will operate as part of the Police’s Counter Terrorist Command unit and will focus primarily on major Metropolitan such as London and Birmingham.
The terror threat assessment level in the UK remains at “severe” — meaning an attack is highly likely with the public urged to be vigilant.
London police will institute road closures in the area surrounding Buckingham Palace when the popular Changing the Guard military ceremony is taking place a security measure rushed into place because of the deadly Christmas market truck attack in Berlin.
Cops say the change in procedure had been planned but is being speeded up. The closures and the new physical barriers are set to be in place by tomorrow.
The Changing the Guard has long been popular with tourists and police say it is a high-profile event that attracts large crowds at an iconic location.
Under the new plan, road closures will be in place for nearly two hours when the guards are changed in front of the palace in central London.
Police said the change is "a precautionary measure" for the next three months and is not based on any specific intelligence.
There was a visible police presence at a number of Christmas Markets across the UK including Bristol and Glasgow.
Officers on horseback were spotted in Bristol.
Police officers were also spotted near Windsor Castle in Berkshire.
Also posters were handed out to members of the public at the Southbank Winter Festival in London .