* All 18 analysts expect stable rates on Oct 5
* Median forecast is no change until Q1 2018 rate increase
* To see results in Eikon app click reuters://realtime/verb=Open/url=cpurl://apps.cp./Apps/cb-polls?RIC=PLINTR%3DECI
By Marcin Goettig
WARSAW, Oct 3 (Reuters) - Poland's central bank is not expected to start raising interest rates until the first quarter of 2018 as economists have downgraded their forecasts for economic growth, a Reuters poll showed on Monday.
Poland suffered its biggest contraction in investment for almost four years in the second quarter of this year, latest data showed, as political uncertainty discouraged firms from spending at a time of reduced European Union aid.
The central bank has kept its benchmark rate at a record low 1.50 percent since last March and will keep it at that level when it meets on Wednesday, according to all 18 analysts polled by Reuters between Sept. 28-30.
A similar poll last month had forecast a first rate rise in the final quarter of 2017 but concerns about the slowing economy has prompted economists to revise their estimates.
"The main reason (we delayed the timing of the expected hike) is that we revised downwards our forecasts for economic growth for 2016 and 2017," said Marcin Czaplicki, economist at Poland's largest bank PKO BP.
PKO BP this month downgraded its 2016 economic growth forecast to 3.3 percent, from 3.5 percent.
"Inflation will not reach the (2.5 percent central bank) target in 2017, perhaps it might exceed 1.5 percent next year," Czaplicki said.
Economists and the government downgraded their forecasts for this year's growth due to the slump in public and private investment.
The finance ministry now expects growth in 2016 at 3.4 percent, while economists polled by Reuters at 3.3 percent.
Most of the rate-setters on the Monetary Policy Council have expressed reluctance to change borrowing costs either way anytime soon.
Economists say worries over banks' profitability and the zloty's exchange rate outweigh concern that deflation, which took hold over two years ago, could damage the economy. (Reporting by Marcin Goettig; Editing by Susan Fenton)