A further two former WorldCom employees have pleaded guilty to fraud at the telecom giant in the centre of a multi-billion dollar fraud scandal.

Betty Vinson, the former director of management reporting, and Troy Normand, director of legal entity accounting, pleaded guilty to securities fraud.

The two are expected to provide evidence against other former company executives, including its former chief executive Bernie Ebbers, as part of the ongoing investigation into WorldCom's collapse.

Four ex-employees have now reached an apparent plea bargain with investigators, whereby a guilty plea is offered along with the expectation of assistance.

Guilty en masse

Ms Vinson and Mr Normand's pleas follow that of former head of accounting, Buford Yates, who earlier this week admitted criminal charges and agreed to help investigators prosecute his former bosses.

Last month, Mr Buford's direct superior, David Myers, pleaded guilty to similar charges, blaming orders from higher up the corporate ladder.

Ms Vinson has reportedly told the judge that in October 2000 she was ordered to falsely report $800m (�512m) in WorldCom's accounts.

She said over the following 18 months she received similar orders to transfer vast sums to expenses to hide the company's growing financial crisis.

Building a case

The most serious charge, securities fraud, could result in 10 years in prison for Ms Vinson, though it believed the sentence could be substantially reduced if she was to provide evidence in the case.

Ms Vinson's lawyer, Joseph Holloman, would not say if she would provide evidence against the company's former chief executive Bernie Ebbers.

But these four guilty pleas are now expected to build a case against WorldCom's former directors, including chief financial officer Scott Sullivan, who is also facing fraud and conspiracy charges.

Mr Sullivan, however, insists that he is not guilty.

Billion dollar case

Lawyers are also thought to be collecting evidence against Mr Ebbers, who resigned in April but has yet to be indicted.

Mr Ebbers has not been indicted

Last month, New York's top law-enforcement official, Attorney General Eliot Spitzer, filed a civil lawsuit against Mr Ebbers as one of four telecoms directors who allegedly received illegal share allocations in newly listed companies.

Mr Myers and Mr Yates were sacked in June after WorldCom revealed a $3.8bn hole in its accounts.

The figure was later revised upwards to $7.2bn.

Prosecutors alleged that Myers and Yates had helped hide nearly $8bn in expenses between October 2000 and August 2001.