Kansas City, Mo. ? Struggling energy company Aquila Inc. announced Tuesday that its president and chief executive officer, Robert Green, had resigned and would be replaced by his brother.

The Kansas City, Mo.-based utility said market conditions have prompted the company to reduce the size and scope of its business. Green is leaving all executive positions with the company and resigning from the board of directors, although he will continue to work with the company during the next 18 months as he explores other career opportunities.

The board assigned Green’s CEO responsibilities to his older brother, Aquila chairman Richard Green Jr., effectively immediately.

Aquila’s stock gained 3 cents to close Tuesday at $4.13 on the New York Stock Exchange.

Aquila, formerly UtiliCorp United, is one of several energy traders that ran into trouble after Enron Corp., the industry’s pioneer, went bankrupt amid a scandal over alleged improper accounting. Aquila had entered the energy trading market the year before Robert Green became CEO, but it announced last month that it would abandon that business.

Richard Green had been chairman and CEO from 1985 until January 2002, when the duties were split and his brother became chief executive.

“Following our decision to reposition the business, both strategically and financially, a much smaller executive team is required,” Robert Green said in a statement released by the company. “I strongly support Rick’s return to leadership of the company as CEO. He has the knowledge to serve our shareholders’ interests well.”

Since May, Aquila has sold millions in assets and cut about 1,200 jobs.

Robert Green’s resignation comes about a month after the company’s credit rating was reduced by two major credit rating agencies. Moody’s Investors Service slashed Aquila’s rating to junk, triggering $192 million in cash and collateral calls. Standard & Poor’s downgraded the company’s debt but stopped one step short of junk status, which spared Aquila from paying $292 million in similar financial penalties.