Immigration staff are failing to take action against hundreds of migrant workers who have no right to stay in Britain, a critical report has found.John Vine, an independent chief inspector of the UK Border Agency, said the visas of migrants whose jobs had ended were not being cancelled.
He also found insufficient checks were being carried out on companies which sponsor overseas workers.
Immigration minister Damian Green said the system was being improved.
The report comes as the government announced non-European Union workers earning more than £150,000 a year and some scientists were to be excluded from the government's immigration cap.
Mr Green said the UK had to attract the brightest and the best to promote recovery.
Mr Vine's report examined the government's points-based system for skilled workers who want to come to the UK.
It found there were 150 cases where the visas of migrants who had finished jobs and were required to leave the UK were not cancelled.
And there were likely to be many more because of a backlog of 3,000 potential cases as yet unprocessed, it added.
Mr Vine said he was concerned the agency was not visiting the firms who hire or sponsor migrant staff.He told the BBC: "The Border Agency needs to do the appropriate checks on the sponsor, and satisfy itself of course that they are a bona fide employer and that their employment exists.
"And that's why I was concerned about the visit to the premises not having been carried out in all cases."
His report also found inconsistent approaches to decision-making, with some applicants refused because of minor omissions, and others given extra time to supply the missing information.
The agency had put profits before securing the UK's borders with an emphasis "always on income-generating work first", Mr Vine added.
And he called for the agency to act promptly to return those living in the UK illegally.
"If people no longer qualify to stay in the UK according to their visa conditions, then they must be required to leave the country," he said.
"Many staff perceived that quality of decision-making and controlling immigration were not as much of a priority for the UK Border Agency as generating income and providing customer service."
'Making it worse' Shadow immigration minister Gerry Sutcliffe said ministers were cutting 5,200 jobs at the agency at the "very time we need to improve enforcement and tackle illegal immigration.
"As the independent chief inspector's report today makes clear, UKBA faces a resources challenge and the government is intent on making it worse," he said.
Immigration minister Mr Green agreed changes needed to be made to ensure the system was effective.
He said: "We do regulate sponsors properly, but obviously I'm very interested in what the chief inspector has to say and, if we do need to improve the UKBA systems, we will do so."
He said communication among agency staff and the IT systems had been improved to ensure the system was moving "in the right direction".