Marriott Corp.'s long-term debt ratings were lowered yesterday by a bond-rating agency that said the Bethesda-based hotel and food company was experiencing delays in selling assets.

A lower rating generally raises a company's cost of borrowing money. But despite downgrading Marriott's debt, Duff & Phelps Inc. of Chicago said Marriott's creditworthiness was still of investment grade, a level required by many major lenders.

Marriott's debt rating is under review by two other major rating agencies, Moody's Investors Service Inc. and Standard & Poor's Corp. The company is facing tight conditions in the hotel industry that have made it harder to sell the hotels it builds.

Securities analyst David Silver of Duff & Phelps said Marriott will sell about $1.2 billion of real estate this year, instead of about $1.5 billion the company had expected to sell at the beginning of the year. Silver said Marriott's sale of its family-restaurant group and its 1,500-room hotel in San Francisco were taking more time than expected, but he expected both to sell by the first quarter of next year.

A Marriott spokesman said the company was disappointed with the downgrade but would not comment further.