Rupert Murdoch's News Corp. said on Monday it is highly unlikely to proceed with its $5 billion bid for Dow Jones & Co. Inc. unless it wins more support from Dow Jones's controlling family.

The comment from a News Corp. spokesman pushed Dow Jones shares down as much 8.5 percent. The stock was down 3.1 percent at $52.76 in early afternoon trading, compared to News Corp's bid price of $60 per share.

The Bancroft family, which controls more than 64 percent of Dow Jones's voting shares, is expected to decide by the end of Monday whether to support the Murdoch bid.

The News Corp. spokesman did not specify what level of Bancroft support is needed, but said it was highly unlikely we would proceed if we didn't get more of the Bancroft family supporting the deal.

A source familiar with the situation estimated between 25 percent and 32 percent of voting shares held by the Bancrofts favored Murdoch's bid, but stressed the situation was in flux. The final tally has not been firmed.

The source said that to be comfortable, Murdoch would probably want a support level of 28 percent to 30 percent. Outside shareholders hold about 29 percent of voting shares and are expected to largely back the deal.

Some Bancrofts want a higher price from News Corp., but the source said Murdoch had indicated he would not raise his bid.

Dow Jones's The Wall Street Journal reported late on Sunday that family members holding about 28 percent of voting power had indicated their support for the deal.

Benchmark Co. analyst Ed Atorino cautioned against making a final call on the deal, noting that several hours remain before Monday's deadline.

Yesterday's vote (tally: Quote, Profile, Research) is not the final vote. Today's vote is the final vote, said Atorino.

Dow Jones shares have slipped from a high of nearly $62 in June on concerns that the deal could fall through. Analysts expect the stock to plummet to levels much closer to the $36 that Dow Jones was trading at before the Murdoch bid was made public.

Dow Jones and Bancroft family officials declined to comment.

The board of Dow Jones endorsed the offer two weeks ago, sending the deal to the Bancroft family for approval.

The family, which has controlled Dow Jones for more than a century, has since been trying to decide among its numerous members whether to support the bid.

Michael Elefante, a Dow Jones board member and chief trustee of the Bancrofts' shares, asked for family members to decide whether or not they support Murdoch's bid by the end of business Monday, the source familiar with the situation said.

Bancroft family members met in Boston a week ago to hear presentations by their advisers on the deal. The Boston meeting also gave members an opportunity to sign an agreement saying how they would vote.

The Bancrofts had also sought rival bids, but the only competing proposal that appears to be on the table is from Internet entrepreneur Brad Greenspan who proposed lending between $400 million and $600 million to some Bancrofts to buy out other family members at $60 a share.

(Additional reporting by Robert MacMillan and Michele Gershberg)