The Stock Exchange is believed to have contacted both the DTI and the MMC to ascertain who has had access to information from the report on National Power's proposed takeover of Southern Electric and that of PowerGen for Midlands Electricity.
The Exchange, which declined to comment, is also expected to voice its concerns to the Takeover Panel following massive surges in share prices in the electricity sector.
The leak caused acute embarrassment in Whitehall and prompted a swift attack by the Labour Party. The MMC document, which appears to give the go-ahead to both takeovers with only minor conditions attached, was submitted two weeks ago to Ian Lang, Secretary of State for Trade and Industry - due back on Monday after a trade mission to Japan.
Mr Lang, who is expected to accept the MMC's advice, was not planning to publish the report for at least another week.
John Battle, shadow energy minister, said the event "throws into question the integrity of the relationship between the Secretary of State and the MMC".
He added: "The contents are so market price-sensitive that energy companies' shares are changing hands before the Secretary of State makes a decision on the findings of the report. This is an incredibly dangerous precedent that casts competition policy aside."
The bids by National Power and PowerGen are controversial as they would in effect recreate the vertically integrated structure of the sector which was dismantled before the industry was privatised.
However, Mr Lang had already set a precedent last year by clearing the takeover of Manweb, an electricity distribution and supply company in the North-west, by Scottish Power. The MMC recommendation to clear the National Power and PowerGen bids will also be seen as a blow to the regulator, Professor Stephen Littlechild, who is against vertical integration. A spokeswoman for the watchdog said that they could not comment on an unpublished report.
The DTI said: "The position remains that no decision has been taken." The Secretary of State must accept an unequivocal MMC approval for a merger. But the DTI said that he could block it if the Commission suggested that a merger might in some way be against the public interest. Mr Lang might also decide to give the proposed merger the go-ahead but to impose conditions of his own.
The MMC is thought to have put forward conditions for the bids including the disposal of any generating plants owned by Southern and Midlands.
National Power and PowerGen are also expected to be forced to ring-fence contracts between the companies they are buying and other generators.
A further constraint would be changes to the companies' licence conditions to ensure that Professor Littlechild has access to information needed to regulate effectively.
Shares in Southern Electric surged by 21p to 893p on anticipation of a renewed bid by National Power while those in Midlands rose by 7p to 405p. Shares in the generators also moved up sharply with National Power gaining 14p to close at 492p and PowerGen up by 13.5p at 562.5p. None of the companies was willing to comment on the report, which they are thought not to have seen.
Industry sources said that National Power and PowerGen are concerned that Mr Lang might impose further power plant disposals in addition to substantial plant sales already being forced on the companies by Professor Littlechild.
City analysts believe that the generators could walk away from their bids rather than see their share of the generating market shrink further.
Separately, it emerged that Prudential has a 4.9 per cent stake in Yorkshire Electricity, which has been at the centre of bid speculation in recent weeks,
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