The Metropolitan Police is considering training public order officers to use firearms to create a "reserve capability" to cope with terrorist attacks.
Met Commissioner Sir Bernard Hogan-Howe confirmed the force is examining options to create additional firearms capacity in the wake of the terror attacks in Paris and the increased terror threat in the UK.
After the 2012 Olympics, Scotland Yard reduced the number of certified firearms officers by around 750 - but senior officers believe a surge in firearms capacity may now be needed.
Police and the security services have long feared a mass-casualty gun and bomb attack - like the atrocity in Mumbai, India, in 2008 - could be carried out in Britain.
The recent attacks by gunmen in Paris, which claimed 17 lives, have increased concern in the UK about the potential for a similar attack this side of the Channel.
The Territorial Support Group (TSG) provide uniformed support across London. They are highly trained officers who specialise in large-scale public disorder, mass protests and major sporting events.
Previously, some TSG officers were firearms trained, helped out with counterterrorism patrols, and provided support to the dedicated firearms unit, SCO19.
However that role ended as the number of firearms officers were reduced in London in the last few years.
Giving evidence to members of the London Assembly, Sir Bernard said it was time to look again at enhancing its firearms capability, adding the force was reviewing its counterterrorism policing levels.
"The TSG were a firearms reserve for a long time," he said.
"They'd been trained for firearms jobs in case we needed a pool of people as a surge capacity.
"Our broad view was that prior to the UK terror threat level being raised to severe, that was something we no longer needed. It is something we will now look at again, post the severe threat assessment."
Sir Bernard also confirmed Scotland Yard is in discussions with the Home Office over the number of people and buildings requiring firearms protection.
"Now that the threat level has been increased, we will have to look again at whether more people and places require additional firearms protection," he said.
But he added any decision on enhanced protection for VIPs and sensitive buildings, which have to be sanctioned by the Home Office, would have to be supported by additional funds from central government.
The Commissioner told Assembly members he estimated tens of millions of pounds would be required to boost the UK's counterterrorism policing capability.
However, he said he was optimistic more funds would be made available by government as part of a recent £130m extra security funding announced in response to the increased terror threat to the UK.