CNN reported no specific location or time frame has been identified but concerns have increased following Wednesday’s decision by British parliament to bomb IS targets in Syria.
British newspaper the Daily Express reported IS is “likely to target Christmas shoppers in its next attack”.
The newspaper quoted Will Geddes, head of global security firm International Corporate Protection: “None of us are safe anywhere unless you are on a remote island”.
Mr Geddes said extremists are planning lone wolf attacks on the streets of London similar to those carried out at the holiday resort of Sousse in Tunisia.
“For groups like ISIS they want a maximum media impact with any horrific attack that has people begging to return to their home nations,” he said.
“We may or may not see those threats moving more inland to major cities as one would expect from any terrorist group. These risks may be moving more into central locations.”
Britain has begun carrying out air strikes in Syria. The Guardian reported the first air strikes targeted oilfields controlled by IS for more than a year.