Charities come in all shapes and sizes and range from household names to organisations you've never heard of. But some of the biggest are also among the most obscure, often because they do little fundraising and therefore have less of a need to interact with the public.
The five charities profiled below are examples of mega-organisations with charitable status that do not fit into the traditional voluntary sector.
The biggest is the same size as Barnardo's, the smallest the same as Christian Aid - but they have far lower profiles than either of those well-known organisations.
Many of these charities were either established by the state or are primarily funded by it, and all but one are health and education organisations; these are activities that require a certain level of scale to be effective.
This selection includes only organisations that are registered with the Charity Commission. If the register also encompassed the UK's 1,000 or so exempt charities, it would be dominated by academies, housing associations and universities, which are regulated by other bodies - and would therefore include many more large charities you've probably never heard of.
Annual income: £253m
The industry training board for the construction sector was set up following the Industrial Training Act 1964 and provides apprenticeships, awards and training to construction firms. Its training services are used by tens of thousands of people each year.
Why a charity? Although it is one of the few industry training boards with charitable status, its objects of education and training allow it to fit into the charity sector.
Why so wealthy? The CITB charges a levy of 0.5 per cent of the wage bills of those construction businesses that have wage bills of £80,000 or more.
Annual income: £176m
The trust provides female-only education throughout England and Wales. The first school was opened in 1873, with 16 pupils, after a meeting in the Royal Albert Hall attended by MPs and peers. It now runs 24 schools and two academies, offering a total of 20,000 places.
Why a charity? There is still much debate about whether independent education should be charitable, but schools remain firmly within the tent. The GDST is the largest of about 2,000 charities that do the same thing.
Why so wealthy? According to its latest annual accounts, the trust receives fee income of almost £9,000 per pupil. It has assets of £256m, including £212m of property, so it seems likely that it will be around for a while.
Annual income: £169m
St Andrew's is a national teaching hospital with four sites in the UK, providing services for people with mental health problems, learning disabilities, autism and brain injuries. It is the biggest charitable provider of mental healthcare.
Why a charity? The provision of healthcare is a charitable purpose. With the growing scope for such provision by charities and social enterprises, there are likely to be more organisations like St Andrew's.
Why so wealthy? St Andrew's receives almost all its income from fees, mostly for people referred to it by the NHS. The charity says it has tripled in size since the 1990s.
Annual income: £107m
The trust is the parent body of the Ormiston Academies Trust, an exempt charity that sponsors 20 academy schools. The Ormiston Trust receives between £1m and £2m of investment income each year.
Why a charity? It gives out grants for charitable purposes. Unusually, because it is the parent charity of the Ormiston Academies Trust, the accounts of the two bodies are consolidated.
Why so wealthy? The academies trust receives government funding to provide education to thousands of children. It is just one of many academy providers that were catapulted into the ranks of the UK's biggest charities in recent years.
Annual income: £101m
Also known as the Sanger Institute, the charity is one of the leaders in the field of human genome research. It also specialises in mouse and zebrafish genetics, pathogen genetics and bioinformatics.
Why a charity? The institute's medical research has life-saving potential, particularly in preventing and curing cancer, and can comfortably be said to be of public benefit.
Why so wealthy? The charity's principal source of funding is the £15bn Wellcome Trust, which provides about 80 per cent of its annual funding.
Annual income: £95m
The facility, which wisely calls itself Iffim rather than using its four initials to form an acronym, is the funding vehicle for the Gavi Alliance, which aims to provide vaccines for 700 million children in the developing world by 2020.
Why a charity? Even though it seems odd to have a bond-issuing mechanism in the charity sector, it seems difficult to argue against the public benefit of providing life-saving health treatments to millions of the world's poorest children. Iffim qualifies.
Why so wealthy? Iffim's income comes mostly in the form of pledges from world governments, which are then used to sell bonds on the public market. The sale of the bonds provides funds for the Gavi Alliance.