A private security company responsible for holding immigration detainees received more than 700 complaints last year, the BBC has learned.
The figures for G4S were disclosed after a Freedom of Information request.
The company runs three removal centres and until recently escorted detainees who were being deported.
A G4S spokesman said it took the complaints seriously, but the level of substantiated complaints was extremely low and they were of a minor nature.
The detailed figures provided to the BBC show that G4S received 773 complaints from immigration detainees in 2010, 240 more than during the previous year.
Almost 640 of the cases were later found to be unsubstantiated and almost 130 were logged as fully or partially substantiated.
The most common complaint concerned lost property or poor communication. There were 48 claims of assault, three of which were upheld. Two complaints of racism were partially substantiated, but six were dismissed.
Quality of service
Overall, the figures released to the BBC show there were 1,497 complaints between December 2008 and April 2011. Eight out of 10 complaints related to the quality of services, rather than allegations of staff misconduct.
The figures show that the proportion of complaints substantiated has fallen over the three years. During the same period, approximately 65,000 people passed through centres run by G4S.
More than half of all the complaints were made at Brook House, near Gatwick Airport in West Sussex, which holds convicted foreign criminals facing deportation.
G4S is one of the largest and most important companies running government services.
It came under the spotlight last year when Jimmy Mubenga, an Angolan detainee, died after being restrained by three guards on a flight. Three men remain on bail while police investigate.
Alongside Brook House, G4S also runs Tinsley House near Gatwick and Dungavel House in South Lanarkshire.
It will soon take on the government's family accommodation unit near Gatwick.
A spokesman for G4S said: "Anyone detained within our immigration facilities is given every opportunity to raise concerns about either the treatment they receive from our staff or the services we provide.
"The level of substantiated complaints against our employees over the last three years, at 25, is extremely low and of a minor nature, albeit we take each one very seriously.
"The vast majority of upheld complaints relate to issues at Brook House IRC, which houses some of the UK's most challenging detainees, including a number of convicted criminals, prior to their deportation."
David Wood, Head of Criminality and Detention at the UK Border Agency said: "The way that complaints are processed and recorded has been made simpler in recent years, which has led to a rise in recording.
"Complaints range from issues about the food served at mealtimes to more serious issues. Importantly these figures show a decrease in substantiated complaints and that the majority of complaints are minor and relate to availability of facilities at the centres."
- Freedom of Information Research by the BBC's Julia Ross.