The memorial to murdered police officer Yvonne Fletcher has been vandalised with white paint.
The plaque in St James' Park London, surrounded by wreaths and flowers, is wrapped in black bin liners following what the police describe as a "despicable act".
This is the latest in a spate of vandalism to statues and memorials in the capital.
The 'Allies Statue' of Winston Churchill and Franklin D Roosevelt in New Bond Street, the Bomber Command memorial in Green Park the Canada Memorial in Green Park and The Royal Marine Memorial in The Mall were all targeted with white paint on Monday.
The police believe all the incidents are linked.
Detective Inspector Dave Watkinson from the Metropolitan Police said: “These crimes have understandably caused anger and offence and we are working hard to identify and apprehend those responsible.
“Our enquiries are moving at pace and I urge anyone who saw anything suspicious at the locations concerned to contact us.”
The RAF Benevolent Fund said it was the fourth time in six years that the Bomber Command Memorial in London's Green Park had been sabotaged and that the white gloss paint remained wet "at daybreak" when it was discovered.
The Police Memorial Trust said they were "disgusted to hear the news of attacks on memorials in London" describing it as a "desecrating of loved ones".
The tribute to Pc Fletcher, 25, stands opposite the former Libyan Embassy where she was shot dead in 1984 while policing a demonstration against Colonel Muammar Gaddafi.
Her death resulted in an 10-day siege of the embassy, at the end of which those inside were expelled from the country and led to the UK severing diplomatic relations with Libya.
Nobody has ever been charged over the killing of Pc Fletcher. An investigation into a suspect, named in reports as Saleh Ibrahim Mabrouk, collapsed after key evidence was withheld from prosecutors on national security grounds.