Senior Eurosceptics are confident that both newspaper proprietors will not only make the issue a personal crusade but will also give "substantial sums" of money to the cause.
The news will be a massive blow to Tony Blair, who had hoped to win over the media to the merits of economic and monetary union before putting the issue to the country.
The newspapers owned by Mr Murdoch and Mr Black, which have a combined weekday circulation of 5.5 million, and a million more than that on Sundays, are now almost certain to campaign against the Government ahead of the ballot.
Mr Black, whose Hollinger group owns the Daily and the Sunday Telegraph, is understood to have already pledged about pounds 50,000 to Business for Sterling, a group of industrialists set up to campaign against Emu, chaired by former Labour transport minister Lord Marsh.
Sources say this is the "first stage" in a series of gifts, and Mr Murdoch, head of News International, which owns the Times and the Sun, recently wrote to Lord Marsh offering support.
In his letter, discussed at a meeting of the organisation's directors last Tuesday, the media mogul asked them to contact Les Hinton, his right- hand man in London. Mr Murdoch has been in discussions for some time with a number of Eurosceptic organisations about a "major donation" to fight Brussels.
Although the stance of the two proprieters is well-known, the extent of their involvement in the anti-Emu drive will infuriate pro-Europe campaigners. Mr Murdoch's letter will scotch rumours that he is ready to drop opposition to the single currency. "He is full-square behind us," one senior Eurosceptic said, "and he has given clear indications that he is ready to give financial support to the Euro- sceptic movement." Eleven of the 15 EU nations switch to the euro in January. The joint effort is further evidence of a growing closeness between the two newspaper proprietors. Friends say they have been in contact since the Telegraph owner made a Eurosceptic speech in July, urging Britain to strengthen its ties with the US rather than Europe.
A spokesman for Mr Black said it was a "personal matter". But a spokesman for News International confirmed last night that Mr Murdoch had written to Lord Marsh, expressing his support. "But it would be totally wrong to suggest he was in any way bankrolling a campaign," the spokesman said.
The support of the media moguls is seen as crucial to combat the single currency. Mr Murdoch and Mr Black were first approached last summer when Lord Cranborne, leader of the Tory peers and a director of the Telegraph, was forming an anti-Emu group. Ministers were concerned by a Sun headline asking "Is this the most dangerous man in Britain?"about Mr Blair.
Business for Sterling has raised almost pounds 1.5m since it was set up this year. Lord Hanson, Sir Stanley Kalms, head of Dixons, and hotelier, Sir Rocco Forte, are among high-profile backers.