LONDON — A stark new prediction that Germany will plunge into recession in the second half of 2012 added significant pressure on the European Central Bank as it took decisive action Thursday to stem the euro debt crisis.


A report from the Organization for Economic Cooperation and Development suggests that the German economy — the powerhouse of the euro zone — will contract 0.5 percent in the third quarter and 0.8 percent in the fourth quarter.


Because of Germany’s much stronger performance in the first half of the year, the O.E.C.D. estimates that, over all, the German economy will grow by 0.8 percent in 2012.


But by highlighting the extent to which the slowdown in the wider European economy is beginning to affect Germany, the estimates by the Paris-based O.E.C.D., which groups 34 developed economies, suggest that the euro zone crisis may have reached a significant moment. Until recently, Germany had avoided the worst effects felt in many other nations in the currency area.


The German chancellor, Angela Merkel — who faces an election next year — has recently softened her opposition to plans by the president of the European Central Bank, Mario Draghi, to help Spain and Italy reduce borrowing costs, which have soared to levels most analysts believe to be unsustainable.


Ms. Merkel was to meet Thursday in Madrid with the Spanish prime minister, Mariano Rajoy.


The report underlined the need for greater coordination at the European level to stem the two-year-old debt crisis in the currency zone.


“With the euro area crisis still the most important risk for the global economy, further policy action is needed to instill more confidence in the monetary union,” said the report, prepared by Pier Carlo Padoan, the O.E.C.D.’s deputy secretary general and chief economist.


Janet Henry, chief European economist at HSBC in London, said that the O.E.C.D estimates were in line with the growing body of evidence indicating that the third quarter of the year would bring the crisis to Germany’s door.


“It’s hard to argue strongly against this type of forecast,” she said. “It is increasingly clear that Germany will see a contraction in the third quarter.”


“We would hope to see strengthening demand from outside the euro zone,” she added. “If not, then the likelihood is of continued weakening in Germany through to the end of the year.”


Ms. Henry said that statistics on gross domestic product alone did not always give a comprehensive indicator of economic sentiment within a country. But she added that “if Germany is feeling the pressure, there will be a greater willingness to support greater easing of policy.”


The three largest euro zone economies — Germany, France and Italy — will together contract 0.2 percent this year, the O.E.C.D. estimates.


Though France’s economy shrank 0.2 percent in the second quarter of 2012, and will decline 0.4 percent in the third quarter, it should record feeble but positive growth of 0.2 percent in the final quarter of the year, and record 0.1 percent growth for 2012 over all, the O.E.C.D. said.


Italy, meanwhile, will be in recession for the whole of 2012, contracting by 2.4 percent, the estimates suggest.


More broadly, the report said the Group of 7 leading industrialized nations would grow by 1.4 percent this year, the same figure as 2011, with the U.S. economy expanding by 2.3 percent.


“The loss of momentum at the G-7 level may persist through the latter half of this year, with the recession in the euro area and associated trade and confidence headwinds enduring,” the report said.


“The United States is an exception, with comparatively stronger growth,” reflecting “progress in balance-sheet adjustment and improving housing market conditions,” among other factors, it said.