Kenneth Lay has been compelled to appear at the hearing

Mr Lay's planned silence has caused anger amonst Enron employees who wants some straight answers from their former manager.

"I feel betrayed by the whole company, but the strongest betrayal is from Mr Lay," said Sherri Saunders, who worked for Enron for 24 years.

Mr Lay has been ordered to testify before the Senate Commerce Committee on Tuesday after he refused to attend a hearing last week.

Mr Lay, who was on Enron's board from 1986 until last month, is believed to have vital knowledge of the earnings statements and complex financial arrangements which have been widely blamed for bringing the firm to bankruptcy.

Enron's collapse was the largest bankruptcy in US corporate history and led to the loss of more than 4,000 jobs.

After a week of sessions, Congressmen said they have strong evidence of illegal activity surrounding Enron's collapse on 2 December.

Panel pledge

Mr Lay's decision follows his withdrawal last week from attending hearings after his lawyer said Congressmen had already decided on his guilt.

The chairman of a Senate panel on consumer affairs, Byron Dorgan, said he was "disappointed" Mr Lay had decided again to remain silent.

"[Mr Lay] has talked for some while about wanting to tell his side of the story to the American people."

The committee would "get to the bottom" of the reasons behind Enron's collapse with or without Mr Lay's co-operation, Mr Dorgan added.

On Sunday, Mr Dorgan had promised the committee would be "respectful but tough" when questioning Mr Lay.

Testimony doubt

Many other former Enron bosses have also declined to testify, including former chief financial officer Andrew Fastow, who earned $30m for running the firm's thousands of partnerships.

But last week Congressmen did cross-examine Mr Lay's former Enron colleague and ex-chief executive Jeffrey Skilling.

Skilling could face perjury charges

Mr Skilling said he did not know about the firm's complex web of partnership arrangements and insisted he had been ignorant of the company's "imminent financial peril" when he left in August.

However, a number of leading Congressmen said on Sunday they did not believe Mr Skilling's evidence.

The chairman of the House Energy and Commerce committee, Republican representative Billy Tauzin, told the US television network CBS that Mr Skilling was "totally incredible".

Mr Tauzin said Mr Skilling could face perjury charges as a result.

Mr Skilling's lawyer, Bruce Hiler, said he was "shocked at the unsupported charges" levelled against his client.