The letter, labeled a "Terrorism/Extremism Update" lists al-Qaeda, FARC and Belarusian bombers as contributing to the "substantial" international terror threat to the UK and adds Occupy London under a "domestic" heading.
It states: "City of London Police has received a number of hostile reconnaissance reports concerning individuals who would fit the anti-capitalist profile. All are asked to be vigilant regarding suspected reconnaissance, particularly around empty buildings."
A City of London police source has confirmed that the letter was circulated to businesses on December 2. A spokeswoman told CNN that the City of London Police has "seen crime linked to protests in recent weeks, notably around groups entering office buildings, and with that in mind we continue to brief key trusted partners on activity linked to protests".
The document was passed to Occupy London protesters at the Finsbury Park encampment over the weekend, and unsurprisingly, the response from the group has been critical.
On the Occupy London blog, the group complains that "activism is not a crime and the desire to participate in democratic decision-making should not be a cause for concern for the police in any free society". It goes on to say that the "document exhibits other signs of worrying paranoia".
City of London Police stress that the letter does not explicitly link the Occupy London group with terrorism but Occupy London continue to call for clarification of the document's language.