* Corn, soybean prices face pressure on favourable US weather

* Ukraine Black Sea grain deal extended for two months

* Crop tour sees lowest southwestern Kansas wheat yield since 2003

LONDON, May 18 (Reuters) - Chicago corn futures slid to the weakest level in 18 months on Thursday as favourable weather boosted crops in the U.S. Midwest and heightened expectations for a bumper harvest this year while soybean prices slumped to a 10-month low.

Dealers said both corn and soybean crops in the U.S. were off to a good start while there was also concern about weak demand, particularly from China.

The most active corn contract on the Chicago Board of Trade was down 1.6% at $5.52-3/4 a bushel at 1214 GMT after setting an 18-month low of $5.52-1/2.

CBOT soybeans were 0.5% lower at $13.30-1/4 a bushel after setting a fresh 10-month low of $13.28-3/4.

Wheat prices also fell as an extension of a deal to allow war-ravaged Ukraine to continue exporting grains through Black Sea ports eased concerns over world supplies.

The Ukraine Black Sea grain deal was extended on Wednesday for two more months, a day before Russia could have quit the pact over obstacles to its grain and fertiliser exports.

ING said in a note that the extension would "help ease some supply concerns in the market" while adding there would continue to be uncertainty about what would happen next.

Ukraine had sought a longer extension while Russia continues to demand more should be done to boost its own exports of grains and fertilisers.

CBOT wheat was down 2.4% at $6.10-3/4 a bushel, after setting a two-week low of $6.10-1/2.

However, losses in the wheat market were limited by expectations of lower winter output in the United States.

Crop scouts on the second day of an annual three-day tour of Kansas projected an average yield for hard red winter wheat in the southwestern portion of the state at 27.5 bushels per acre, the worst since at least 2003 and down from 37 bushels per acre last year. (Additional reporting by Naveen Thukral in Singapore; Editing by Mark Potter, Kirsten Donovan)