Widespread racism was exposed inside Britain's immigration removal centres last night in an official report.

The Borders and Immigration Agency (BIA) said its detainees had been called "black bastards", "donkeys" and "animals" and described the atmosphere as "distressing and turbulent" at one centre and "tense" at another.

The agency, part of the Home Office, is responsible for holding thousands of failed asylum seekers and other offenders and criminals being thrown out of the UK but contracts out the management of its removal centres to a variety of private and state bodies.

It launched its official audit of racism - published yesterday - after a TV documentary broadcast in 2005 uncovered racist abuse at the Oakington removal centre in Cambridgeshire.

However, BIA praised Scotland's only centre, Dungavel in Strathaven, South Lanarkshire, for its work with minorities, despite some evidence of racism.

The agency found the worst racism at Harmondsworth, a large centre near Heathrow frequently used to house former Scottish residents on their way out of the country.

"Regular taunting of detainees by some officers goes unchallenged," the BIA report found. Repeated patterns of alleged racist incidents were missed by the in-house investigation process, it added.

There were even "staff on staff" problems at Harmondsworth, where an auditor overheard a senior officer tell an Asian colleague: "Talk proper, I can't understand you."

Rating each of the 10 centres with a series of performance scores, last night's report, carried out by an independent team from Focus Consultancy, ranked Harmondsworth - run by private company Kalyx - as the worst, with a percentage score of 35%. The two best centres, in Dover (77%) and Haslar, Hampshire (81%), were run by the state. Dungavel was among the best with 69%.

"The centre has a very tranquil feel and most of the detainees seemed comfortable in their surroundings," the report said of the controversial Lanarkshire facility. "The approach by staff was very low-key, and, whether by accident or design, it was a style very suitable to the Dungavel environment."

The centre did not, however, escape criticism. It won plaudits for its "cultural kitchen", where detainees can share dishes from their home countries, but auditors discovered some residents felt they were denied access to the kitchen "as a result of bias or the racist attitude of staff".

The Herald last year revealed insider concerns about the number of ex-convicts awaiting deportation from Dungavel, which also houses the families of failed asylum seekers.

Ex-cons are filling many of the UK's other nine removal centres. At one, Colnbrook, which, like Harmondsworth, is near Heathrow, they told auditors they would rather be in jail than in immigration detention.

It was at Colnbrook that investigators heard a member of staff refer to detainees as "black bastards" and shout: "Animals, lock-up time."

BIA chief executive Lin Homer said: "The BIA takes any allegation of abuse or misconduct very seriously which is why we commissioned this report.

"We are pleased the report recognises race relations procedures are operating above average at nine out of 10 centres. Our aim, however, is further improvement and to that end we will consider the recommendations closely."