By Gerrard Raven

LONDON (Reuters) - European Union finance ministers agreed a plan to shore up defences against future credit market turmoil on Tuesday in the wake of reports that the turbulence has led the IMF to cut its 2008 economic growth forecasts.

Ratings agency Standard & Poor's warned that the U.S. subprime mortgage crisis, which caused interbank lending almost to seize up, would not peak until 2009.

And the British government applied more sticking plaster to Northern Rock NRK.L, the highest profile victim of the credit crunch in the European Union, offering to guarantee new retail deposits in the bank.

The International Monetary Fund had slashed its forecast for growth in the United States next year to 1.9 percent from a 2.8 percent projection on July 25, IMF sources said.

The IMF has cut its forecast for world growth next year to 4.8 percent from 5.2, for the euro zone to 2.1 from 2.5, and for Japan to 1.7 from 2.0, they said.

"The risks are firmly on the downside, based on the fear that the tensions on financial markets could increase and cause an even more marked global slowdown," Italy's ANSA news agency quoted the IMF as saying.

EU finance ministers agreed to review a series of financial rules to apply lessons from the summer's credit squeeze.

"We agreed on a work programme, a so-called road map, laying down what we will do through 2008," Fernando Teixeira dos Santos, finance minister of EU president Portugal, told a news conference after a meeting in Luxembourg.