Soldiers could fill in for armed police after scores of Metropolitan Police officers stood down from firearms duties following a murder charge against one of their colleagues.
Scotland Yard requested military support for counter-terrorism duties if armed officers are unavailable.
Home Secretary Suella Braverman has ordered a review of the situation, insisting officers “mustn’t fear ending up in the dock for carrying out their duties”.
More than 100 officers have reportedly handed in permits allowing them to carry weapons, prompting Scotland Yard to turn to the military for assistance.
The crisis has emerged after an unnamed officer was charged with murder over the shooting of unarmed Chris Kaba, 24, who was killed in September last year in Streatham Hill, south London.
A Met Police spokesman said: “The Ministry of Defence has agreed to a request to provide the Met with counter-terrorism support should it be needed.
“This is a contingency option that would only be used in specific circumstances and where an appropriate policing response was not available.
“Armed forces personnel will not be used in a routine policing capacity. We will keep the need for the support under constant review.”
The Ministry of Defence said: “We have accepted a Military Aid to the Civil Authorities (MACA) request from the Home Office to provide routine counter-terrorism contingency support to the Metropolitan Police, should it be needed.”
Mrs Braverman said she had ordered a review to ensure armed officers “have the confidence to do their job”.
The Home Secretary said: “We depend on our brave firearms officers to protect us from the most dangerous and violent in society.
“In the interest of public safety they have to make split-second decisions under extraordinary pressures.
“They mustn’t fear ending up in the dock for carrying out their duties. Officers risking their lives to keep us safe have my full backing and I will do everything in my power to support them.”
A Met Police officer appeared in court on Thursday in relation to the fatal shooting of Mr Kaba.
Senior officers, including Scotland Yard chief Sir Mark Rowley, have been meeting with firearms officers in recent days to reflect on the murder charge.
Scotland Yard acknowledged the situation had prompted “a number of officers” to “step back from armed duties while they consider their position”.
The Met is supporting officers who have stepped back from firearms duties to help them “fully understand the genuinely held concerns that they have”.
Mr Kaba, 24, died after being shot through an Audi car windscreen.
The officer accused of his murder, named only as NX121 after a district judge granted an anonymity order, appeared at Westminster Magistrates’ Court and the Old Bailey on Thursday.