Italy's Eni backs carbon tax - CEO

* Carbon tax to help make investment decisions

* Cap-and-trade systems to be integrated later

MILAN, Sept 22 (Reuters) - Industrialised western countries should introduce a carbon tax as a key way to curb CO2 emissions more efficiently than existing cap-and-trade systems, Italian oil and gas major Eni's ENI.MI chief executive said on Tuesday.

Western countries, major energy consumers and carbon emitters “should be the first to go on a diet” and should start with a small carbon tax and a complimentary mobile excise tax on energy products, Eni CEO Paolo Scaroni said in a speech for a climate change forum in New York.

“By attributing a stable cost to CO2, a carbon tax immediately affects investment decisions. It would need to be accompanied by measures to offset its effects on income distribution,” Scaroni said in the speech.

A text of the speech was provided by Eni.

“Cap-and-trade systems -- complicated mechanisms which require many years of trial-and-error processes -- could be integrated in time, with the aim of optimizing the effects of the carbon tax,” Scaroni said.

France is set to be the biggest European economy so far to adopt a carbon tax after President Nicolas Sarkozy this month launched plans for such system to encourage industry and households to cut energy consumption. [ID:nLA40561]

Some Nordic countries adopted carbon taxes in the 1990s and said the measures helped cut emissions without crippling growth.

Scaroni also proposed a “mobile excise tax” on end-use energy products derived from fossil fuels to be applied when product prices fall below the level that boosts investment in research and development and energy efficiency.

“It would shelter environmental policies from energy price volatility,” Scaroni said, adding that emerging countries should to agree to similar measures “after a reasonable grace period.” (Reporting by Svetlana Kovalyova, Editing by Peter Blackburn)