Aiming to curb air pollution in central London, motorists in Westminster who sit in a stationary car with the engine running and refuse to turn it off will be fined 20 pounds from May 1.
The move reflects concerns from local residents and businesses about air quality, Westminster City Council said.
A team of traffic marshals will patrol the borough and ask car idlers to switch off their engines, it said.
The penalties will come into force on May 1, as a "last resort".
The new policy follows Camden Council's fines for buses which idle for too long, introduced in 2011, and Islington Council's fines for idling vehicles introduced in August last year.
Westminster has the highest proportion of deaths attributable to air pollution, excluding the City of London, in the country, a report by the council said.
The London Taxi Drivers' Association (LTDA) said the move to impose a fine of 20 pounds would not help abate London's air pollution problem.
Steve McNamara, general secretary of the LTDA said the fines were "absolute tosh" and a public relations, money- raising exercise from the council.
"It is not going to do anything constructive to tackle the pollution crisis in London. When was the last time you saw someone idling in the city? It doesn't happen," he was quoted as saying by the BBC.
"One of the real problems is cars stuck in traffic - research has shown pollution is up by 30 per cent in areas of heavy traffic. Do something to help get the traffic moving."
McNamara said another key factor was the number of diesel cars in the capital, which are worse for the environment.
He said the move was "ironic" considering chancellor George Osborne's incentives to diesel vehicles which were exacerbating the problem.
Neil Greig, director of policy and research at the Institute of Advanced Motorists, said the "key issue" would be how the marshals enforced the fines.
He said: "If they target private car drivers pulling up for a moment in relatively clean, modern cars in a draconian way - whilst ignoring old buses, coaches, large trucks, utility company vans or taxis idling for long periods - then it will quickly be seen as yet another revenue-raising exercise.
