* ECB holds rates at 1 percent for fourth month running
* Trichet sees very gradual recovery from recession
* ECB staff upgrade economic forecasts
* ECB keeps rate on 12-month finance at 1 percent
(Adds Stark comments, market reaction)
By Marc Jones and Sakari Suoninen
FRANKFURT, Sept 3 (Reuters) - The European Central Bank kept interest rates unchanged at a record low of 1.0 percent on Thursday, and warned that now was not the time to withdraw support as economies emerge slowly from recession.
President Jean-Claude Trichet said the euro zone faced a very gradual recovery and stressed it was too early to end exceptional ECB measures to boost the economy. ECB staff also raised their estimates for euro zone economic output this year and next, while inflation estimates were edged higher.
Trichet announced the ECB would pump more one-year funds into the financial system at just 1 percent interest later this month - a decision financial markets took as a sign rates would remain on hold for an extended period.
Asked about how long the ECB would keep up its ultra-loose policy, he told a news conference: "Today is no time to exit."
"It is more of a bumpy road ahead. Uncertainty is high. Prudence and caution are still of the essence."
Interest-rate sensitive 2-year euro zone yields fell to a six-month low and the euro dropped as investors locked in expectations that the ECB will stick to its very accommodative monetary policy, and showed relief the central bank would not add a spread when it offers the next 12-month funds.
"By keeping the spread at zero over refi it keeps the ECB positioned to hold rates low for the time being," said Peter Chatwell, fixed income strategist at Calyon.
Trichet stuck to the line that the euro zone's record-low interest rates remained "appropriate", bolstering expectations that the ECB won't raise them until the third quarter next year at the earliest.
All 80 economists polled by Reuters before the decision had expected a fourth month of rates on hold and Sweden also kept rates unchanged earlier on Thursday.
Asked if the decision on the 12-month refi operation meant rates would stay on hold for the next year, Trichet said the ECB was not precommitted.
STAFF FORECASTS
Against a backdrop of encouraging data in recent weeks, ECB staff lifted their economic forecasts.
Germany and France, the region's biggest economies, unexpectedly bounced out of recession in the second quarter. The overall euro zone economy shrank only 0.1 percent against the previous three months, following a 2.5 percent drop in January-March.
New quarterly staff projections put gross domestic product growth in the 16-nation region at between -0.5 and +0.9 percent next year, giving a midpoint of +0.2 percent, compared with a range of -1.0 percent to +0.4 percent in the June forecasts.
ECB staff also upgraded their projections for this year, and said they expected GDP to fall between 4.4 and 3.8 percent in 2009, a slightly smaller contraction than the 5.1 to 4.1 percent range given in June.
But economists worry that the momentum will be lost once central banks and governments around the world begin to take away the emergency support measures built up during the crisis.
They have also raised doubts about the speed and sustainability of any recovery because unemployment, which reached 9.5 percent in July, is at the highest in more than 10 years.
Trichet said the ECB expected inflation to remain subdued and price stability to be maintained over the medium term, and was guarded on growth.
"There are increasing signs of stabilisation in economic activity in the euro area and elsewhere," he said on the economic outlook. "This is consistent with the expectation that the significant contraction in economic activity has come to an end and is now followed by a period of stabilisation and very gradual recovery."
The need for caution on the nascent economic recovery was underscored as ECB policymakers met. Euro zone retail sales fell unexpectedly in July, data revealed, a reminder that demand remains fragile at best.
ECB Executive Board member Juergen Stark said it was too early to sound the all-clear.
"Uncertainty is very high, one cannot assume that we are on the path of sustainable recovery," Stark told a conference.
TENDER INTEREST
Trichet confirmed the ECB would offer banks unlimited 12-month funds at a flat rate of 1.0 percent at an operation later this month, opening the door to repeated strong demand.
In late June, the ECB flooded markets with a record of 442 billion euros of one-year funds, which they are still digesting, in an effort to kickstart lending.
The ECB had previously said two other such operations, scheduled for September and December, could be at a higher rate. But Trichet said: "This decision (to keep the rate at 1 percent) should promote the extension of credit to the euro area economy and therefore further underpin its economy."
Economists expect the ECB will bump up its lending rate as one of its first steps to unwinding all the emergency support that it has put in place.
Trichet sidestepped repeated questions on whether the ECB had considered increasing the price of one-year funds this time around, or whether it was considering doing so in December. (For a graph of world interest rates, please double-click on: http://graphics.thomsonreuters.com/RNGS/ECO/RATES.jpg) (Reporting by Marc Jones, additional reporting by Kirsten Donovan in London; editing by David Stamp/Ruth Pitchford)