Firearms officers have been ordered "go forward" to confront terrorists before treating the wounded in the event of a Paris-style attack in the UK.
The new tactic means the wounded may be left untreated as the attackers were pursued, but could end up saving more lives.
It comes as armed police took part in a dramatic training exercise yesterday simulating gun-wielding terrorists storming a shopping centre.
Assistant Commissioner Patricia Gallan, Scotland Yard's head of Specialist Crime and Operations, insisted the force has no "shoot-to-kill" policy and said marksmen will operate only within the law.
She said: "We know from the incidents in Paris and elsewhere, that the terrorists have taken a different tack in that they have been shooting hostages and such like.
"So the decision has to be made, and it is a brave decision, about whether you stand back, in which case you may well have more people die, or whether the best thing is to go forward.
"And we've been training our officers to go forward. That is, going forward in the face of firearms and shots being fired at them as well as potential explosions and such like.
"We believe that is potentially what will save the most lives."
More than 2,000 authorised firearms officers took part in the mock terror attack on a London office block.
Around 25 masked and armed officers then entered the building in waves to clear the site and confront the gunmen.
With the air spiked with the smell of gunpowder they were faced with casualties littered throughout the block, a home-made bomb and the gun-wielding "terrorists" themselves.
Crews from London Ambulance Service and Fire Brigade also took part in the roleplay.
Although it bears chilling similarities to the attacks in Paris, police said the training exercise had been planned for months before the tragedy happened.
Ms Gallan added: "In light of Paris we still thought it would be appropriate to be open, to show what was happening.
"I hope it will reassure people that we are really committed to keeping the capital safe and mitigating the threat of terrorism."