(UPDATED) The MoD said on Sunday 24 Sept that it had accepted a request from the Home Office, under Military Aid to the Civil Authorities (MACA), to "provide routine counterterrorism contingency support to the Metropolitan Police, should it be needed".
The linkage of the request to counterterrorism support excluded the use of armed defence personnel for armed police duties not connected to counterrorism.
The Met said the next day that the Army had been 'stood down' because enough of their own officers had returned to firearms duty.
This followed a number of authorised armed Met Police officers stepping back from armed duty following the CPS decision to prosecute one of their number following the shooting of Chris Kaba last year.
The Home Secretary has ordered a review into armed policing, saying that armed police must not fear "ending up in the dock for carrying out their duties".
It is assumed that any troops allocated to support the Metropolitan Police under the MACA request would have been specialists already trained in and available for counterterrorism operations, and would be given all necessary support including legal/ROE updates. It would not have involved the routine sight in the streets of armed tactical troops.