Share a Capital Christmas, which helps children, families and the elderly over the festive period, supplying toys and non-perishable foods, will become part of Help a London Child, the grant-giving organisation that funds children's play and helps children with special needs.
Capital FM DJ Chris Tarrant has agreed to become patron of Help a London Child for a year and David Attenborough takes on the role of honorary patron.
Consultant Lolly Walters, who has been auditing the two charities over the past nine months, is acting as director until a person with a voluntary and commercial background can be recruited to the position full time.
Walters said: "The two charities were sitting right next door to each other but weren't maximising their opportunities. By merging the charities they are in far better shape to develop corporate relationships."
A company has also been set up, Capital Charities Trading, to sell items such as Christmas cards, the profits of which will go to Help a London Child.
George Michael, Robbie Williams, Ronan Keating and David and Victoria Beckham, along with Toyota and London Electricity, all support the charity.
The Beckhams are giving away two tickets on Capital for their pre-World Cup party.