The donation made by the Paul Hamlyn Foundation (PHF) will be used to fund an endowment, enabling the charity to scale up its work on delivering children's rights.
The newly-created Hamlyn Endowment for the Rights and Voice of Children will also help the charity increase its capacity to provide legal advice for children and families who struggle for representation.
Coram has been supported by PHF for more than two decades after it awarded its first grant to the charity in 1997.
Dr Carol Homden, Coram's chief executive officer, said the landmark gift from PHF was a reflection of its commitment to marginalised children and young people.
"We are creating the space to bring together people from all walks of life to champion a better world where all children have the chance and the resources they need to live a fulfilling life," said Homden.
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She added: "Sadly today, still too many children and young people in the UK do not get the help and support they need, and this generous gift from the Paul Hamlyn Foundation will enable us to advance the Coram children's rights centre and to realise our ambitious plans for the future."
Homden said the endowment would enable Coram to work with hundreds more young people and build their own ability to act as advocates, as well as directly benefiting thousands more through an increased provision of legal advice.
Moira Sinclair, PHF chief executive, said of Coram, established as a charity in 1739: "It's a service they have been offering for hundreds of years and one that speaks to many of our areas of interest, including giving young people a voice, supporting migrants, social justice, arts and access to education."
She said PHF understood how valuable long-term, consistent and flexible funding could be to charitable organisations.
"At a time when civil society is under financial stress, and the world around us feels increasingly fractured, we feel the need to act, to show our backing for some key organisations, ones we see as critical to and influential in the sectors we support, and it is for this reason we have made this decision to endow Coram," she added.
Coram Children's Legal Centre (CCLC) and Coram Voice are among a number of charities that form the Coram Group, supporting more than 100,000 children, families and professionals each year.
The charity's work to advance children's rights during 2019 includes a report setting out the risk of a second "Windrush" generation of EU national children and young people as an unintended consequence of the government's EU settlement scheme.
It also published the first study of care leavers' wellbeing in March and held the first national creative writing competition for children in care and young care leavers.