Jeff McMahon testified he had raised concerns about Enron's dealings

Enron, based in Houston, Texas, said it was Mr McMahon's decision to leave and that his position would not be filled until closer to the launch of the remodelled company.

Mr McMahon is said to have openly questioned some of the deals and partnerships which led to Enron filing the largest bankruptcy in US history last December.

He is also one of the current and former Enron executives accused of getting rich on stock sales when they allegedly knew the company was on the verge of collapse.

According to lawsuits, he sold 39,630 Enron shares for $2.7m from October 1998 to November 2001.

Mr McMahon had been working with interim chief executive Stephen Cooper on a plan to reorganise Enron.

He issued a statement saying: "Making room for new leadership before we announce the plan for the new company is best for Enron's stakeholders and for me."

Conflict of interest fears

A trained accountant, Mr McMahon started his career at Arthur Andersen, which is itself facing problems after becoming embroiled in the scandal as Enron's auditor.

He joined Enron in 1994, working under the then chief financial officer, Andrew Fastow.

He testified before the US Congress that he was concerned about being caught in a conflict of interest with Mr Fastow.

He had to negotiate deals on behalf of Enron while Mr Fastow had to do the same thing on behalf of partnerships in which Enron hid billions of dollars of debt.