SYDNEY (Reuters) - QBE Insurance Group Ltdsaid it expects to post a $250 million net loss this year due towritedowns and unexpectedly large claims after weak crop prices hitits U.S. operations, sending shares in Australia's biggestinsurer down 20 percent.

The profit warning was the latest in a string of earningsdisappointments from QBE, which has completed more than 75acquisitions in the past 10 years to expand to 50 countries andgrappled with hefty claims from at least one major market for eachof the past few years.

The warning and resulting sell-off drew comparisons to goldminer Newcrest Mining, which similarly suffered a slew of profitwarnings after writedowns on previous deals.

“It's a horrible situation, it's akin to what we have seen fromNewcrest over the years,” said Chris Weston, an analyst at IGbased in Melbourne.

“If you are an investor, you are looking at consistentdisappointment from the management level.”

QBE said it was putting aside more money for claims made a yearearlier from things such as workers compensation and constructiondefect risks, while its North American crop insurancebusinesshad also suffered.

“The record crop yield projected by the U.S. Federal Governmenthas not materialised, increasing QBE's exposure to revenue claimsas a result of the collapse in crop prices (particularly for corn)after early season preventive planting claims erodedthe Federal Crop Insurance Corporation reinsurancedeductible,” QBE said.

Corn futures fell to a three-year low in November as a bumperU.S. crop and a proposal to lower the use of corn-based ethanol inthe United States dragged on prices.

QBE, which posted a net profit of $761 million a year earlier,said on Monday it expected a net loss of around $250 million for2013. It forecast a cash net profit after tax of around $850million for 2013. This is down from $1.04 billion the year before,and about 28 percent below analysts' forecasts, according toThomson Reuters data.

QBE, which generates about 30 percent of its revenuefrom North America, said it had already put in place a newexecutive team for the region.

The insurer separately announced that Belinda Hutchinson, adirector for 16 years, would step down as chairman in March 2014and be replaced by board member Marty Becker.

The downgrade followed a review of QBE's North Americanoperations that prompted it to increase its provision for prioraccident year claims and write down goodwill, intangibles and otherassets.

The revision was largely due to a $300 million claims increaseand $330 million in write-offs of identifiable intangiblesassociated with QBE's financial partner services businessin North America, it said.

QBE also cut its an insurance profit margin to around6 percent for 2013, from a previous guidance of 11 percent.

Shares in the insurer, which were placed on a trading halt lastFriday, were trading down 19 percent at A$12.50 at 0130 GMT, afterearlier hitting A$12.25.