Chris Phillips, the former head of the National Counter Terrorism Security Office, urged UK citizens to keep on their guard this weekend, and for the next month.
He told the Daily Star: “ISIS positively encourage followers to concentrate their efforts during this time.
“I would encourage everyone to be vigilant and report any suspicions to the police.”
Today marks the beginning of the Islamic holy month of Ramadan, where Muslims across the globe will fast from sunrise to sunset, to celebrate the month the Koran was first presented to the Prophet Muhammed.
However, in recent years extremists have used the peaceful occasion as an opportunity to carry out brutal attacks.
In 2015, more than 400 people were killed in ISIS attacks in France, Kuwait, Syria, Somalia and Tunisia – all on the first Friday of Ramadan.
The first Friday of the month of fasting is considered the holiest – and Mr Phillips fears more attacks will take place this year.
He said: “It’s all to do with the first Friday of Ramadan being the holiest day and most appropriate for terror attacks.