WASHINGTON - Former Enron chairman Kenneth Lay is asserting his right against self-incrimination and will refuse to answer questions when he appears before Congress under subpoena this week.

"Under the instruction of counsel, Mr. Lay will exercise his Fifth Amendment rights at today's hearing," Lay's spokeswoman, Kelly Kimberly, said last night in Houston. She declined further comment.

Lay, who resigned Jan. 23, was a friend and political backer of President Bush and moved easily within the corridors of power in Washington. He has not spoken publicly about Enron's collapse since the company entered the biggest bankruptcy in U.S. history in December, and lawmakers had hoped for his testimony.

Following an intense week of hearings, leaders of some of Congress' investigations say they have strong evidence of illegal activity surrounding the failure of the big energy trading company.

After Lay backed out of a voluntary appearance a week ago, two congressional committees issued subpoenas compelling his presence. Some lawmakers had said they expected that he would assert his Fifth Amendment right against self-incrimination, though his attorney had not previously indicated he would do so.

At the White House, spokesman Ari Fleischer said yesterday that Lay's decision to invoke his right to silence was "a reflection of how seriously Ken Lay and others at Enron take the administration's criminal investigation."

"It's not for me to speak to the reasons (Lay) gives. I can only speak to the seriousness with which our administration is investigating," he said. He mentioned inquiries by the Justice Department, the Labor Department, the Securities and Exchange Commission and a variety of congressional panels.

Lay joins four other current and former top Enron executives, including ex-chief financial officer Andrew Fastow - the mastermind behind the complex web of partnerships that brought the company to ruin - who last week refused to answer questions.

David Duncan, the lead Enron auditor fired by the accounting firm Arthur Andersen for destroying Enron-related documents, also has taken the Fifth Amendment before Congress.

"I'm disappointed (Lay is) going to do that," said Sen. Byron Dorgan, D-N.D., chairman of the Senate Commerce subcommittee on consumer affairs. "He has talked for some while about wanting to tell his side of the story to the American people ... Ultimately, we will get to the bottom of this, with or without his cooperation."

Lay has been subpoenaed to appear today before the full Commerce Committee and on Thursday at a hearing of the House Financial Services subcommittee on capital markets.

Lawmakers say they have not considered granting immunity from prosecution to Lay or other executives in return for their testimony, because they do not want to interfere with the Justice Department's criminal investigation of Enron.

Congress can compel witnesses to show up, but cannot force them to answer potentially incriminating questions without granting them immunity from criminal prosecution.

Lay's wife, Linda, said recently there were some things about Enron's finances her husband wasn't told about by other company officials.

Former Enron chief executive officer Jeffrey Skilling did testify last week, although several lawmakers said yesterday they didn't believe the sworn testimony of Skilling. One suggested that Skilling could face accusations of perjury as a result of his testimony.

A House investigative panel heard hours of conflicting testimony last Thursday from Skilling and other top Enron officials. Skilling said he knew few details of the 4,000 or so partnerships used to keep some $500 million in debt off Enron's balance sheet and hidden from investors and securities regulators. He insisted he was never warned of problems with the arrangements.

Rep. Jim Greenwood, R-Pa., chairman of a House Energy and Commerce subcommittee, and Rep. Billy Tauzin, R-La., who heads the full committee, said on CBS' "Face the Nation" they didn't believe Skilling. They cited instances in which he said he couldn't recall details of key conversations on Enron's finances that subordinates testified they had with him.

"He was totally incredible," said Tauzin. "This is the guy who was in charge of the corporation ... I'm afraid he may have put himself in some legal jeopardy as a result."

Asked whether Skilling could face a perjury indictment by federal prosecutors, Tauzin replied: "That could happen. I mean you can't come to Congress ... take that oath as he did in front of Jim Greenwood, and then not tell the truth ... He could have some real problems."

Skilling's attorney, Bruce Hiler, said in response, "There is no basis for the allegations being made concerning my client's testimony."

"Mr. Skilling has cooperated with regulators and Congress and we're not going to respond to attempts to twist that testimony and the testimony of other witnesses to fit the predetermined view of some," Hiler said in a statement.