Sometimes described as the "biggest company you've never heard of", Serco was in celebratory mood just over a year ago, with profits surging after it had won a record number of contracts ranging from community healthcare in Suffolk to retaining the joint running of Britain's nuclear warheads store at Aldermaston.

But everything changed for the giant outsourcing conglomerate when it became mired in a series of scandals, saw its stock market value slump and was ignominiously ejected from the FTSE 100 index.

Nevertheless, it still employs 120,000 people in more than 30 countries, and, once you start looking, you will see its logo everywhere.

In Britain the group runs a huge range of government and other services, from rail franchises including Merseyrail, Northern Rail and London's Docklands Light Railway, to immigration removal centres and prisons – including the Thameside prison in south-east London that was criticised last year by the Howard League for conditions that were "truly alarming".

It has won a number of healthcare contracts as private involvement in the NHS increases, such as out of hours GPs in Cornwall – damned in a report by the House Of Commons public accounts committee that was prompted by reports in the Guardian and found Serco had replaced clinicians' on out-of-hours GP services in Cornwall with call-handlers without medical training.

It looks after school inspections in the Midlands and east of England. It manages the UK's ballistic missile early warning system on the Yorkshire moors and is in charge of traffic monitoring on England's motorways. In London it runs the "Boris bikes" cycle hire scheme. It also operates the National Nuclear Laboratory.

Around the world, Serco has hoovered up government and private contracts ranging from air traffic control in 11 states of the US and in the United Arab Emirates to parking-meter services in Chicago and driving tests in Ontario. In Australia it runs Great Southern Railways as well as ports, patrol boats and immigration services including an immigration centre on Christmas Island. Other big transport contracts include toll tunnels in Hong Kong and the metro in Dubai.

It provides IT support for the European Commission, the European Space Agency and Italian regional and central government; and engineering services at the European Particle Physics Laboratory (CERN) close to Geneva.

Serco began life in 1929 as a UK subsidiary of US electronics company Radio Corporation of America, providing services to Britain's burgeoning cinema industry. In the 1960s it won a maintenance contract from the Royal Air Force base at Fylingdales, which expanded into a bigger support contract with the RAF. Following a management buyout in 1987, RCA Services Limited was renamed Serco and a year later, it listed on the London Stock Exchange.