Investors, understood to include the Prudential, are critical of the full Panel's inconsistency in overruling its executive and extending the bid's closing datefor four days until Christmas Eve.
The extension cost Northern its independence. In the event, however, only just over 20 per cent of outside investors accepted the bid on top of the 30 per cent CalEnergy already owned.
"You should not change the rules of the game as it is being played, which the Panel did. There was a fair degree of muddle in all this, which stopped investors getting a fair price," one leading investment manager said.
Yesterday the Prudential, which intervened to support Northern, said it had yet to decide whether to accept the 650p bid.
M&G and Foreign & Colonial are among the institutions sore at the Panel's decisions. The Prudential will also make strong representations to the Panel's own inquest of events.
"We regret the fact that a management that did well by shareholders was not successful in these exceptional circumstances," a Prudential spokesman said.
The Panel is already considering a substantial alteration of its rulebook following the controversy which centres on its broker BZW's role in the defence. BZW and Schroders, Northern's merchant bank, bought a 2.4 per cent stake in their client to stop CalEnergy getting over 50 per cent acceptances before the bid expired on Friday 20 December.
The full Panel subsequently extended the bid after BZW's late disclosure of a pounds 250,000 performance-based fee for its work. That raised suspicions, denied by all parties, that BZW was being improperly remunerated by Northern to buy its own shares.
The Panel will now review whether advisers will be allowed to buy shares in future at all in a decision due by mid-January. Some members of the panel are in favour of a total ban to bring the UK in line with US practice. Institutions including the Prudential are also pressing for full disclosure of advisers' fees.
"Our aim has always been to have an open and liberal policy, and not to seek to fetter markets unduly," said Alistair Defriez, the Panel's director general. "In the past we have allowed purchases of shares by advisers as long as they were on an arms-length basis and with no financial inducements. The situation with Northern has raised questions over whether we should prohibit these purchases,"