Counter-terror police are investigating a possible threat against serving officers in what has been described as a "fast-moving and dynamic" inquiry.

Officers and staff with West Midlands Police were called in to special briefings by their bosses overnight and security measures stepped up after the force received an anonymous tip-off.

Police would not confirm the specific details, but did not deny reports that there had been a threat to kidnap and kill a police officer.

West Midlands Counter Terrorism Unit (WMCTU) is now leading an investigation into the tip-off.

Assistant Chief Constable Garry Forsyth said the force takes any threat to its officers' safety seriously but added that it is "business as usual".

He said: "We take any information in relation to the security of our staff and the public very seriously.

"It's really important to stress there is no information we've received that indicates any threat to the public, and this relates purely to officers and staff.

"We've taken appropriate measures to ensure our staff have the information they need to continue to provide the business they do, day in and day out.

"But in terms of specific information that's not something I'll talk about in detail."

The briefings advised officers not to come to work or go off shift in uniform, and also included a reminder about basic police station security measures.

Mr Forsyth said every officer on operational duty had been given "basic and fundamental security advice", adding that otherwise it was "no different to any other day".

He would not be drawn on how the tip-off was made, or what it contained, saying only that the information was "relating to the safety of West Midlands Police officers and police staff".

The information is still being assessed, the force has said.

Metropolitan Police Commissioner Sir Bernard Hogan-Howe said the force had spoken with their colleagues in the West Midlands and believed they had received the tip-off through "an anonymous call".

He said the Met had not given any guidance to its own officers, but was monitoring the situation.

Mr Forsyth would not reveal the precise nature of the anonymous contact, adding: "All I would say is this is subject to a fast-moving and dynamic investigation by our counter-terrorism officers right now, so I won't comment on the specifics.

"It's business as usual."

He moved to reassure the public, saying officers were out solving crimes despite the heightened level of security.

"You can be reassured that they are out there - they are going about their normal daily business," said Mr Forsyth.

"They're dealing with road traffic collisions, involved with safeguarding children and investigating serious crime, and that is happening now.

"It is no different to any other Tuesday and it was no different last night to any other Monday."

Meanwhile, Sir Bernard said Scotland Yard was carrying out its own assessment of the information received, adding that the Met tried not to "overreact" in situations like this.

He said: "One of my colleagues has talked to the Chief Constable of West Midlands (Chris Sims) to find out exactly what the state of the threat is.

"And once we've assessed it, we'll decide whether or not to give any further advice."

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.

This latest security alert 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.

In 2008, Islamist fanatic Parviz Khan was jailed for life with a minimum term of 14 years over a brutal terror plot to kidnap and behead a British Muslim soldier.

British-born Khan, of Foxton Road, Alum Rock, Birmingham, had planned to lure a soldier off the streets with the promise of drugs before filming the killing.

The 37-year-old admitted the kidnap plot at Leicester Crown Court, also pleading guilty to supplying equipment to terrorists and two counts of possessing records or documents likely to assist terrorists.