$54.6bn offer marks most serious assault to date on Michael Eisner's leadership of the Magic Kingdom.

Comcast unleashed a bold step in the consolidation of the global media business with a $54.6 billion hostile bid for Walt Disney.

The offer marks the most serious attack to date on Mr Michael Eisner, Disney's chairman and chief executive, and his stewardship of the Magic Kingdom.

The bid, outlined yesterday in a letter to Mr Eisner from Mr Brian Roberts, the US cable operator's chief executive, comes as Disney faces attack from two former directors over its management record while nursing the collapse of its partnership with Pixar, the successful animation studio.

The bid is the latest in a series of huge deals this year, with January and February set to see mergers and acquisitions worth $365 billion, greater than in any year since 2000.

In banking, Bank One is to merge with JP Morgan; in pharmaceuticals, Sanofi-Synthelabo has launched a hostile bid for Aventis; and the telecoms industry has been convulsed by the auction of AT&T Wireless, which is to close tomorrow.

If successful, the deal would create the world's largest media group by market capitalisation, combining leading brands such as Disney, ABC and ESPN with a distribution platform that reaches 21 million US subscribers.

For Comcast, the offer is a potentially risky step in a history of dealmaking that has propelled the group from humble origins in Tupelo, Mississippi to become the dominant US pay-TV operator.

It comes two and a half years after it launched a hostile bid for the cable operations of AT&T, the ailing telecom group.

Quickly integrating that deal, approved by regulators at the end of 2002, set the stage for its latest move.

Mr Roberts said: "We think we're ready for the logical next step and that of course is entertainment and content."

Comcast is also making its move when rivals are unlikely to be in a position to respond. Mr Rupert Murdoch said yesterday the combination of Comcast and Disney would stand with News Corp and Time Warner atop the US media market.

But the News Corp chairman, who last year bought a controlling stake in satellite service DirecTV, said: "there's going to be one hell of a row in Washington".

There are few immediate overlaps between Comcast and Disney but the deal marks the latest stage in the vertical integration of the US media business when regulators and legislators are warning about big media.

After some preliminary talks between intermediaries, Mr Roberts called Mr Eisner last Monday to outline his proposal.

Mr Roberts said Mr Eisner responded he was not interested.

Disney said its board would "carefully evaluate" the offer.

Mr George Mitchell, Disney's senior non-executive director, is expected to defend the company's strategy at an investor conference in Orlando today.

Shares in Comcast, which is offering 0.78 of its Class A voting shares for each share in Disney, were down 7 per cent by early afternoon in New York, eliminating much of the premium it had offered. Disney shares jumped 13 per cent to $27.32, indicating investors are expecting Comcast to raise its bid.

Merrill Lynch analyst Ms Jessica Reif Cohen called the proposed merger a "perfect, brilliant, combination," noting Comcast's ability to squeeze value from previous mergers. But she cut her rating on Comcast to "neutral" from "buy," saying its shares would be under pressure.

Federal Communications Commission chairman Mr Michael Powell said the proposed deal would get "ruthless and rigorous scrutiny".

Mr Roberts, however, said the companies were not competitors and predicted a merger could be completed in a year or less.

The Comcast bid comes as Mr Roy Disney, the nephew of company founder Walt Disney and a former board member, ramps up his bid to unseat Mr Eisner, whom he accuses of mismanaging the company over the last decade and draining Disney's creative strength.

Mr Roy Disney declined to comment on the Comcast offer, which was made independently of his anti-Eisner campaign.

Disney yesterday reported that its earnings for its fiscal first quarter ended in December rose to $688 million, or 33 cents per share, from $36 million or 2 cents per share a year earlier, topping the Wall Street consensus of 23 cents per share.