By Jeremy Lovell

LONDON, Feb 10 (Reuters) - Owners of gas-guzzling cars could be paying 25 pounds ($50) a day for the privilege of driving them in central London from October if mayor Ken Livingstone decides this week to go ahead with the plan as expected.

Livingstone, who floated the idea over a year ago, is to announce his decision on Tuesday.

It is part of a package of measures that Livingstone, who has made the environment a central plank of his tenure and who is facing a tough re-election battle in May, may bring in to cut London's carbon emissions by 60 percent within 17 years.

"The rest of the world is watching to see if we can deliver on this incredibly ambitious target of a 60 percent reduction by 2025," Mark Watts, Livingstone's climate chief, told Reuters.

London, which generates some seven percent of Britain's climate-warming carbon emissions, is in the vanguard of a group of 40 major cities worldwide pooling their knowledge to play their part in fighting climate change.

The city's plan is far more ambitious than legislation going through parliament to cut national emissions of the main greenhouse gas carbon dioxide by 60 percent by 2050.

Ordinary cars driving into central London's Congestion Charge zone has to pay eight pounds a day and the 25 pound daily tax would apply to vehicles emitting over 225 grammes of carbon dioxide per kilometre.

But to force home the environmental point of a congestion scheme that initially had no green goal, an exemption granted to residents in the zone will be removed from drivers of the polluting four-wheel drive and top-end luxury cars.

That means that the owner of a gas-guzzler who drives in the zone every day will end up paying 6,500 pounds a year.

"It is the first instance of real carbon pricing being applied to people's everyday existence," said Watts. "This is getting much closer to the idea of a personal carbon budget."

The attack on gas guzzlers -- known as Chelsea Tractors because of the large number driven by residents of that rich central London borough -- is only one of several key measures.

A low emission zone targeting heavy lorries came into force on Monday in the 1,579 square kilometres area inside the M25 ring road circling the sprawling city.

Livingstone is also expected next week to announce a major step forward in his campaign to get cars off London's roads with a boost for bicycling in and around the city -- including copying Paris' successful roadside cycle hire scheme.

Added to that, he is also due to announce a comprehensive plan to fit new filters and equipment to all municipal buildings in the city to cut their carbon emissions.

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