Mr Paddick says the patrols could distance the police from the people
Routine armed patrols in London pose "obvious dangers", ex-deputy assistant commissioner of the Metropolitan Police Brian Paddick has warned.
A trial is taking place in three areas, which senior officers say is to tackle a 17% rise in gun crime since April.
Speaking on the Today programme, Mr Paddick said: "How do they (criminals) know which police officer has a gun and which doesn't have a gun?"
He also warned that the measure could "distance" the public from the police.
"There is a tradition of policing in the UK, policing by consent, not by force, policing with the support and the co-operation of the public," said Mr Paddick, the Liberal Democrat candidate in the 2008 London mayoral elections.
"And there is a danger of further distancing the police from the public.
"Particularly in relation to gun crime, the police rely on information from the public to be able to deal with this particular offence," he told the BBC Radio 4 programme.
"The other danger with this, of course, is that these armed officers are working alongside unarmed community officers.
"And if there is an escalation, if criminals start to carry guns because the police are carrying guns, how do they know which police officer has a gun and which doesn't have a gun?"
Officers from the CO19 unit have begun pilot patrols in Tottenham, Brixton and Haringey.
The Met said the measure was a temporary response to rising gun crime.
On Thursday Ch Insp Neil Sharman, of CO19, said the move was a "more proactive approach to deal with weapons on our streets".
A spokeswoman for London mayor Boris Johnson said the use of armed police "should be the exception not the norm".
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