Police officers have been warned not to wear their uniforms while travelling to and from work over fears they could be targeted in a terrorist attack.
The warning from MI5 arrived after four men were charged with plotting to kill police officers in a terrorist attack.
The Association of Chief Police Officers (ACPO) also warned officers to be vigilant "for their own personal safety" following a rise in the police's internal threat level from "moderate" to "substantial".
Home Secretary Theresa May also announced in August the terror threat level in the UK had risen to "severe", suggesting an attack is "highly likely".
Now senior officers at the Metropolitan Police, which oversees national policing matters, are telling officers to keep a low profile outside work and instructed them not to post personal information about themselves on social media.
A police source told the Mirror: "A raised threat level means officers have to be extra vigilant in everything they do. It means not travelling to work in full uniform and standing out unnecessarily out of work.
"Officers are also being warned about putting sensitive information about their personal lives on social networking sites such as Facebook, Twitter and Instagram."
The unnamed source added officers are frustrated that they have to arrive early for work in order to change into their uniforms.
The source added: "Many uniformed officers would turn up at the station just before their shift started in full uniform.
"Now they have to spend time in the locker room getting ready, which adds time onto their average working day for no extra pay."
The instructions echo similar guidelines given to soldiers following the murder of Fusilier Lee Rigby in Woolwich last May.
The instructions, which were later rescinded, said that "uniforms should be concealed while in a public place, particularly when travelling alone or from home to unit". Rigby was killed in a daylight attack near his barracks by Michael Adebolajo and Michale Adebowale while wearing a Help for Heroes hooded top.
MI5 are now investigating reports Islamic terrorists are monitoring the social media accounts of police officers in order to help launch an attack.
Four men, Tarik Hassane, 21, Suhaib Majeed, 20, Nyall Hamlett, 24, and Momen Motasim, 21, are due to stand trial accused "retaining images" of police officers during a plot to kill them in a suspected drive-by shooting at Shepherds Bush Police Station.
Security expert Neil Doyle said: "It makes perfect sense that terrorists would be using social media to scout for targets. Other criminals such as burglars are known to monitor online chatter to see who in a particular area might be out for the evening or has gone on holiday.
"They can then break into those houses knowing that the risk of them being discovered by the occupants is minimal."
An ACPO spokesman added: "We are constantly reviewing our security and safety procedures to ensure that police officers and staff have appropriate and up-to-date guidance on how to keep themselves safe from harm.
"We are asking them to follow existing policies and good practice. Our emphasis is on vigilance and remaining alert to any possible dangers, on and off duty, which includes personal use of social media."