G4S boss Ashley Almanza told MPs yesterday that the company had trouble telling right from wrong.

Being grilled by public accounts committee MPs about overcharging for tagging offenders, Mr Almanza said: "I don't think we did correctly tell the difference between right and wrong. We got it wrong."

And fellow privateer Serco chairman Alistair Lyons admitted that it was "ethically wrong" that his company also milked the Ministry of Justice for millions.

The Serious Fraud Office (SFO) is investigating the two firms for overcharging on the tagging contracts - claiming money for offenders who turned out to be dead, back in prison or overseas.

The pair apologised and insisted they weren't involved in any other dodgy dealings.

But public-sector union Unison leader Dave Prentis stormed: "Saying sorry hardly compensates for a catalogue of catastrophes and for wasting millions of pounds of taxpayers' money.

"How many more millions will be wasted by failing contractors before the government starts hearing the alarm bells and calls for a wider review of the use of outsourcing in public services?"

Committee chairwoman Margaret Hodge said the firms had only stopped because they were caught.

"If you had never been caught on some of these people who are out of jail, dead or whatever, you would have carried on charging to the year 3,000," she said.

The government has rejected a £24 million offer from G4S to settle the overcharging scandal.

Officials vowed to "pursue all possible avenues" to recoup the cash.

Asked if anyone had been sacked, Mr Lyons said: "The Serious Fraud Office has asked us not to potentially compromise their investigation by our own at the moment."

Mr Lyons agreed the same applied at Serco.

Both firms' share prices have plummeted in the wake of the scandal, with Serco losing a third of its value - equal to £1.3 billion.