Cancer Research UK has been handed its biggest-ever donation of £10 million, the charity said.
The donor, who wishes to remain anonymous, also handed £10 million to the University of Southampton.
A spokesman said it is the largest one-off donation the charity has ever received.
The money is to be put towards Cancer Research UK's Create The Change campaign, which will raise £100 million for the Francis Crick Institute in London.
The institute, which will open in 2015, will be the largest biomedical research building in Europe.
The donation to the University of Southampton is to be put towards building a cancer immunology centre that will have links to the institute.
Cancer Research UK's chief executive Dr Harpal Kumar said: "We're extremely grateful for these extraordinarily generous donations.
"The gift for the Francis Crick Institute accounts for 10% of the total we need to raise and takes us a huge step closer towards achieving our ambition of creating this world-class institute to help us beat cancer."
Professor Don Nutbeam, vice chancellor of the University of Southampton, added: "This remarkable gift allows us to build on our expertise and expand the research teams in Southampton to make even greater progress in developing new treatments."