Former Enron chief executives Ken Lay and Jeffrey Skilling have been directly implicated in the collapse of the energy giant in December 2001 by disgraced former chief financial officer Andrew Fastow.

Giving testimony at the trial in Houston, Fastow told government prosecutors that he thought he was being a hero when he inflated company profits.

'At the time, I thought I was helping myself and helping Enron to make its numbers,' Fastow said.

He alleged both Lay and Skillling approved the off-the-books partnerships he managed and were aware they were used to hide billions of dollars of debt.

Furthermore, Fastow alleged a $16m (£9.2m) partnership he initially created to help Enron manipulate its earnings was replicated on a much larger scale at Skilling's urging, and eventually raised $386m (£222m) from investors to buy Enron assets.

Fastow has already pleaded guilty to two counts of conspiracy to commit fraud and will serve a 10-year sentence under an agreement with the government. He also agreed to forfeit nearly $28m (£16m) of the money he received at Enron.

Lay, 63, and Skilling, 52, have denied any wrongdoing in the collapse of the company and have blamed Fastow for Enron's demise.