* reuters://realtime/verb=Open/url=cpurl://apps.cp./Apps/cb-polls?s=GCR01+AL+0&st= M e n u
+ C for ECB monetary policy data
* reuters://realtime/verb=Open/url=cpurl://apps.cp./Apps/econ-polls?RIC=EUGDPQQP for GDP data
BENGALURU, Sept 15 (Reuters) - The European Central Bank will announce in October a six-month extension to its asset purchase programme but will cut how much it buys each month to 40 billion euros from January, a Reuters poll of economists found.
The euro zone has been performing stronger recently than at any time since the global financial crisis. That has fueled expectations that the ECB will begin scaling back its quantitative easing (QE) programme after more than two years of bond buying during it which snapped up more than 2 trillion euros worth of mainly government bonds.
The Reuters poll of ECB watchers, taken Sept. 11-14, predicted that the resurgence in economic growth will be maintained over the coming year. But inflation is not expected to reach the central bank's target of close to but just below 2 percent until 2019 at least.
Mounting optimism about the euro zone economy has led to a run-up in the euro exchange rate, posing a dilemma for the central bank as it tends to tamp down imported inflation.
ECB President Mario Draghi said at his September news conference that the central bank was looking at how to wind down its 60 billion euros of monthly asset purchases and would be ready with a plan by October.
While a Reuters poll on Sept 1 also flagged October as the most likely date, economists are now nearly unanimous. Only two of 52 surveyed thought the ECB would wait until December, when the current programme is scheduled to end.
The ECB is expected to announce a six-month extension to QE, according to the consensus of 39 economists who answered an additional question, taking the program through to the end of June. Forecasts ranged from 3-12 months.
"The tricky exercise for the ECB is to actually announce that they will continue buying in 2018, but that it will peter out gradually these purchases," said Peter Vanden Houte, chief euro zone economist at ING Financial Markets. "They have to present it in a way that it is perceived by the markets as dovish instead of hawkish."
But the decision to start scaling back bond purchases soon is also a practical one: It already owns a huge chunk of the euro zone government bond market.
"A continuation of QE in unchanged form into 2018, as has been rumoured in some quarters, does not appear to be an option...as bond supply scarcity poses a hard technical constraint on the QE program," noted Marius Gero Daheim, senior eurozone strategist at SEB.
Asked what will be the ECB's monthly purchase amount from January, the median response from a similar number of respondents was 40 billion euros, down from the current 60 billion. The range of views was 30-50 billion.
Asked when the ECB would close the program completely, 31 of 33 economists said by the end of next year, including six who said it could be wrapped up in the first half of 2018. The remaining two said the purchases would end sometime in 2019.
Most forecasters who were asked if the ECB was right to start unwinding its ultra-easy policy said it should. But the vast majority also said the ECB would not likely do anything with its key interest rates through to the end of next year.
The euro's rally this year - up over 13 percent against the dollar and hit multi-year highs after the Sept 7 meeting - has some ECB policymakers worried, suggesting the pace of any reduction in QE is likely to be slow.
Draghi mentioned the currency's strength as a concern several times at the September press conference. But ECB board member Benoit Coeure said earlier this week that the euro's strength may have less of an impact than in the past.
A Reuters poll of foreign exchange strategists published on Sept. 7 predicted the euro will hold on to most of those gains over the coming year. But they also said if the euro rose another 5 percent, the ECB would become uncomfortable.
Economic growth in the current quarter was forecast in the latest Reuters poll at 0.5 percent, up from 0.4 percent predicted last month. Median forecasts show steady 0.4 percent growth through to the end of next year.
While economies rarely grow at such a steady rate for that long, the stability in euro zone growth forecasts, which for years were nearly universally pessimistic, is a positive sign.
Inflation is forecast to remain well below the ECB's target until at least 2019, averaging 1.5 percent this year and 1.4 percent next. Only one forecaster had inflation at 2 percent at any point through the end of next year.
(For other stories from the Reuters global long-term economic outlook polls package)
(Polling by Indradip Ghosh and Sujith Pai; Editing by Ross Finley and Hugh Lawson)