Tycoon Sir Richard Branson is giving £100,000 to a fund to assist Kate and Gerry McCann with their legal costs.
Madeleine McCann has been missing since May
His spokeswoman said: "He wants to give them a chance to have a fair hearing."
The McCanns, from Leicestershire, have been named suspects in the inquiry into the disappearance of their daughter Madeleine in Portugal on 3 May.
They are being advised by Michael Caplan QC, who fought efforts to extradite General Pinochet. Sir Richard is asking other rich people to donate.
"Over the last few weeks Richard has been watching events as they have unfolded," said his spokeswoman.
"There is a whole family involved here. When the McCanns made it known that under no circumstances would they touch the Find Madeleine fund, and discussed selling their house, Richard felt something had to be done."
Branson flew Gerry McCann to the US on one of his planes
Sir Richard is a father himself and the most important thing for him is that a four-year-old girl is missing, the spokeswoman added.
"If he can help a little bit to take the burden off the family and extended family in this small way, then that's all to the good."
Mr McCann, a consultant cardiologist, is on unpaid leave and his wife has confirmed she does not yet plan to return to work as a locum doctor.
The McCanns have said they will not use any of the £1m "Find Madeleine" fund to finance their legal costs.
Sir Richard has kept in touch with the couple since Madeleine disappeared from their holiday apartment.
His Virgin Group companies have helped the McCanns over the last four months.
Virgin Atlantic flew Gerry McCann out to the US to talk to child abduction experts, and Virgin Mobile sent texts to their customers in Spain and Portugal asking them to keep looking for Madeleine, shortly after she disappeared.
The help with legal costs comes a day after the McCanns announced another advertising campaign to publicise the disappearance of their daughter.
Up to £80,000 from donations to the Madeleine Fund will be used for newspaper, television and billboard adverts beginning in a fortnight.
The support from Sir Richard also coincides with the decision by life-long friends of Kate McCann to publicly defend her reputation as a mother.
Linda McQueen and Nicky Gill, who have known Kate McCann since childhood, gave an interview saying Kate and her husband Gerry were "the most loving, family-oriented couple".
Meanwhile, the McCanns attended Sunday mass at their local church in Rothley, Leicestershire, for the first time since returning from Portugal on 9 September.