* Global growth forecast raised to 3.9 pct for 2018, 2019
* Spectre of a trade war looms over improved outlook
* Fed seen raising interest rates four times in 2018
* Tax cuts boost U.S. outlook, UK lags all G20 economies
PARIS, March 13 (Reuters) - The global economy will see its strongest growth in seven years in 2018 thanks to a rebound in trade and investment, the OECD said on Tuesday, while also warning a trade war could threaten the improved outlook.
Updating its outlook for G20 economies, the Organisation for Economic Cooperation and Development raised its global growth forecast for both 2018 and 2019 to 3.9 percent - the highest since 2011 - from a previous estimate of 3.6 percent for both years.
The raised forecast was in part due to expectations that U.S. tax cuts would boost the world's biggest economy, it said.
"We think that the stronger economy is here to say for the next couple years," acting OECD chief economist Alvaro Pereira told Reuters. "We are getting back to more normal circumstances than what we've seen in the last 10 years."
Rebounding global business investment would keep global trade growth at about 5 percent this year, the OECD forecast.
However, it said the global economy was vulnerable to an eruption of trade tensions after the Trump administration slapped import tariffs on steel and aluminium.
"This could obviously threaten the recovery. Certainly we believe this is a significant risk, so we hope that it doesn't materialise because it would be fairly damaging," Pereira said.
For a table of forecasts:
The OECD forecast the U.S. economy would grow 2.9 percent this year and 2.8 percent in 2019, with tax cuts adding 0.5-0.75 percentage points to the outlook in both years.
Against that backdrop, the Federal Reserve would probably have to raise interest rates four times this year as inflation picked up, Pereira said. Previously the OECD had estimated three hikes would suffice this year.
Among other hot economies, stronger growth in France and Germany boosted the outlook for the broader euro zone to 2.3 percent for this year and 2.1 percent in 2019. Previously, the OECD had forecast 2.1 percent and 1.9 percent respectively.
Fiscal easing in Germany's coalition agreement was seen lifting growth in the euro zone's biggest economy to 2.4 percent this year (+0.1 percentage point) and 2.2 percent in 2019 (+0.3).
President Emmanuel Macron's social welfare, tax and labour market reforms would help France narrow the gap with Germany, with growth forecast at an 11-year high of 2.2 percent (+0.4) before easing to 1.9 percent in 2019 (+0.2).
Britain, however, was seen missing out on the global upturn, lagging all other G20 countries with growth of only 1.3 percent this year, although up from a November forecast of 1.2 percent due to the broader global improvement.
With Britain due to leave the European Union next year, its economic growth was seen easing to 1.1 percent in 2019, unchanged from the OECD's November estimate. (Reporting by Leigh Thomas; Editing by Richard Lough and Susan Fenton)