The Telegraph: A security alert was triggered after a threat was made to kidnap and kill a serving West Midlands Police officer, according to reports.
The force said it had taken the step of holding officers back on duty to give special security briefings to police and staff overnight, implementing what it called "additional security measures" following an anonymous tip-off, Sky News said.
Those measures included advice to officers about travelling to and from work in partial uniform and police station security.
Reports of the threat comes after five men were charged in October with plotting to kill police officers or soldiers on the streets of London.
Members of the gang allegedly swore allegiance to extremist group Islamic State and scouted out Shepherd's Bush police station and White City Territorial Army Barracks on a "hostile reconnaissance" mission on Google Street View.
They allegedly kept Instagram images of two Scotland Yard police officers and two Metropolitan Police community support officers, as well as a trove of jihadist material including videos of beheadings.
West Midlands Police said the information which triggered the alert was still being assessed.
Assistant Chief Constable Garry Forsyth said "officers remain on patrol" while staff were working "as usual".
However, he said the force had given officers a security "reminder" in light of the tip-off.
"We have taken the opportunity to remind all employees of the need to be vigilant," he said.
The force has not yet officially said how it received the information, or precisely what the tip-off contained, but Mr Forsyth said: "West Midlands Police treat all information relating to matters of security extremely seriously.
He added: "In this particular case we have received information relating to the safety of West Midlands Police officers and police staff.
"There is nothing at this time to suggest there is an increased risk to members of the public."
In October the threat level to police officers across the country was raised to substantial, meaning a targeted attack is a strong possibility.
The national security level remains at severe, meaning a terrorist attack is highly likely.