* Final decision due on Aug. 18
* Fifteen ships held up by customs
(Adds ship held up by customs, quotes, details)
By Pavel Polityuk
KIEV, Aug 12 (Reuters) - The Ukrainian government is considering limiting combined exports of wheat and barley in the 2010/11 season, while the Customs Service has already blocked shipments from ports, traders' union UZA said on Thursday.
Ukraine, the world's largest exporter of barley and sixth-largest of wheat, is due to make a final decision on export quotas Aug. 18 after severe winter frosts and a scorching summer heat wave ruined tracts of its crops.
"According to the information we have, the government plans to introduce a quota on wheat and barley," Volodymyr Klymenko, the head of UZA union, told a news conference.
"We have heard about the figure of 5 million tonnes for wheat plus barley."
Klymenko estimated that Ukraine could end up with enough stocks for exports of around double that amount this season, or 5 million tonnes of wheat and barley each. In the 2009/10 season, which runs from July to June, Ukraine exported 9.2 million tonnes of wheat and 6.2 million of barley.
He said exports of maize were likely to be free this season. Ukraine exported 5.3 million tonnes of maize in 2009/10.
The government, which last month said that there was no ground to limit exports, this week raised concerns over bread shortages after the farm ministry cut the 2010 wheat crop forecast to 17 million tonnes from 20.9 million in 2009.
15 SHIPS HELD AT PORT
Traders have complained that the customs services virtually halted exports from Ukraine this month after a surge in domestic grain prices risked triggering rises in bread price.
On Thursday the Customs Service said it had prevented export of 28,500 tonnes of wheat because of incorrect paperwork.
"Ukrainian grain traders gave the customs (service) export documents that contained wrong information," the customs service said in a statement.
Most of the cargo was on one vessel, though the customs action also involved a smaller cargo on a second ship. It was not clear what the destination of the grain had been.
Klymenko said at least 15 grain vessels were in Black Sea ports awaiting permission to leave.
"There are about 15 ships in ports, and it is unclear whether they will leave Ukraine or not," Klymenko said.
Klymenko said traders needed a clear decision by the government because more than 1 million tonnes of grain were in ports' silos or going to ports by railroad.
In July Ukraine exported 764,103 tonnes of grain, including 224,630 tonnes of wheat and 414,372 of barley, far below the level of July 2009 at 1.684 million tonnes, including 626,744 tonnes of wheat and 704,943 of barley.
Ukraine traditionally exports barley in the first months of the season.
In 2009/10 Ukraine sold abroad a total of 21.5 million tonnes of grain compared to 25 million in 2008/09. (Editing by Jane Baird)