Prudential was required to publish a formal announcement to the Stock Exchange admitting it inadvertently e-mailed a confidential internal document to a number of analysts.
A member of staff working in investor relations mistakenly sent the group's internal business plan to a handful of analysts who receive data electronically.
The Stock Exchange prohibits the release of price-sensitive information to only a select band of investors. The rules are meant prevent people dealing on the basis of privileged information.
The business plan gave the group's hoped-for sales and closely resembled the new business figures the member of staff had meant to send. If investors had dealt on it they would have struggled to make money - it was three months out of date.
No one noticed the error until one analyst rang to query the figures. The news wires were saying the Pru's premium income had jumped 32 per cent, but these figures were lower; evidently the Pru had sold more pensions than expected.
"You can quote me saying 'Whoops'," said a spokesman for the Pru. "We had to put out an announcement because people might base decisions on it. But it was only part of a draft business plan. This was really just an administrative error and there was nothing too sensitive in it. It was just a handful of analysts who saw it - those who receive electronically. I've got no comment on heads rolling for it."
The man from the Pru - who recently adopted the slogan: "Getting the right mortgage for you is like nailing jelly to a wall. Right, let's nail some jelly" - is not blushing alone.
ICI was recently suspended by the Stock Exchange after accidentally breaking the same rules. The group told New York investors about a US disposal but accidentally failed to tell counterparts in London. The Exchange stopped traders placing orders at 8.50am; by 9am ICI had been forced to correct the error.