UK Border Agency staff processing visa applications from Africa are "acting unfairly" and wrongly refusing entry to the UK, an independent report claims.
Chief inspector to the UKBA, John Vine, said many visas were incorrectly rejected after employees "disregarded or misinterpreted" evidence.
Some applicants were being refused for failing to provide information that had not been originally requested, he said.
The agency said it took the findings "seriously" and would improve training.
Mr Vine's inspection ran between May and July 2011, examining the applications handled at four UKBA posts in South Africa, Nigeria, Kenya and Croydon, south London - which deals with Algerian-based applications.
He reported claims by UK Embassy staff in Algeria that the visa delays had caused "reputational damage" to the UK.
And in a review of 135 visa applications from Nigeria, his report found the UKBA had made a "serious error" after 14 cases had accidentally received "indefinite leave to enter the UK" - rather than the usual 27-month limit.
In some cases, it found UKBA staff had failed to retain documents backing up their decisions.
Various reasons for visa rejections were listed in the report. In one case, an applicant wishing to visit his uncle in the UK was turned down because they had different family names.
Visas to the UK enable successful applicants to stay and work in the country for two years, with an option to apply for permanent residency at the end of that period.
UKBA's visa bases in Abuja, Nigeria; in Pretoria, South Africa; and Nairobi, Kenya had "performed well" to meet customer service targets, Mr Vine found.
But staff at Croydon's Visa Section in the UK had been "poor" in processing applications made in Algiers, Algeria.
Mr Vine said that little progress had been made by the UKBA in a number of areas, even though recommendations had been made in previous inspections.
"This is especially frustrating", he said, "considering the agency has accepted the recommendations, and yet I continue to identify the same issues.
"I would now like to see these recommendations being embraced by the agency without delay to ensure that there is a real improvement in the quality and consistency of decision making."
A UKBA spokesman said the agency was prioritising improvement.
He said: "We take the independent chief inspector's findings seriously and are making reforms, which include providing detailed guidance to applicants and improving the training for staff handling visa applications.
"The UKBA must offer a high quality service for genuine applicants while ensuring that those who do not meet the immigration rules are prevented from entering the UK," he added