The Metropolitan Police commissioner, Sir Bernard Hogan- Howe, said the move was a direct response to the Paris terror attacks last November, in which 130 people were killed by terrorist gun attacks and suicide bombers.
"It will be an expensive option, but is vital to keeping us safe.
"My firearms officers are our heroes - we expect them to run towards a terrorist attack and take action to confront and stop that threat. By increasing the number of armed response vehicle officers we can make sure that our firearms response continues to come from a group of highly specialist and highly skilled officers," he said.
Mayor of London Boris Johnson welcomed the move, saying it was "absolutely essential".
Also Read
"While this city remains one of the safest in the world, the terrible events in Paris last year remind us that we have to be prepared to meet any potential terrorist threat," he said.
The Met stressed thatit would not change the "fundamental principle" that police in the UK are not routinely armed, with about 92 per cent of the Met officers still unarmed.
Prime Minister David Cameron has also ordered a review of the use of guns by police in England and Wales, following the Paris attacks.
Armed officers from Scotland Yard and several other forces have been holding regular training exercises at an undisclosed location in central London in preparation for the possibility of a mass firearms attack in the UK.
In recent years, police tactics have evolved to counter the changing nature of the terrorist threat.
The change in tactics is a response to the knowledge that terrorists from groups like Islamic State use negotiations only as a stalling tactic to allow them to kill as many innocent people as possible.