Plans to arrest suspected illegal immigrants had to be delayed because of a lack of detention space, inspectors said today.
John Vine, the independent chief inspector of the UK Border Agency (UKBA), said four of six operations planned by one of the agency's 53 arrest teams had to be re-scheduled due to a "lack of detention space".
There was no room to hold any more men when inspectors assessed the team working in Croydon on February 8 with just 24 hours' notice.
"The remaining four operations were due to be re-scheduled when detention space became available," Mr Vine said.
"However, as intelligence reports are only valid for a period of three weeks, according to agency guidance, there is a risk that this period might be exceeded.
"In addition, the efficiency of the team can be compromised when four arrest team visits are cancelled."
A second report into the agency's use of intelligence found the UKBA fails to keep track of how it deals with immigration offences highlighted by tip-offs from members of the public.
Frontline staff at the UK's ports also used different methods to spot suspicious people or vehicles, but had no way of telling which of these best identified potential offences or offenders, Mr Vine said.
He said it was unacceptable that despite receiving more than 100,000 allegations every year from members of the public, the agency was "unable to identify the proportion of allegations that had resulted in people being prevented from entering the UK, or which had led to enforcement action against people living or working illegally in the UK".
Mr Vine also found inconsistent views on whether the UKBA "could or should be intelligence-led".
"There is a real need for the agency to focus more rigorously on the actual outcome of intelligence," he said.
"There is insufficient understanding across the agency of the role that intelligence should play and whether or not it is the driving force for meeting objectives.
"The agency should have a clearer idea of how the use of intelligence contributes to preventing and detecting immigration and customs offences."
The short-notice inspection of the agency's arrest team in Croydon also found "significant non-compliance with the agency's own policy and guidance", both in the briefing before the visit and during the operation, Mr Vine said.
The errors led to unacceptable "significant failings" in its operations and a "potentially considerable" obvious risk to staff and members of the public.
Intelligence checks were carried out two months before the operation and it was unclear whether they had been re-checked since, the report said.
"This lack of a clear audit trail presents an obvious risk; for example, if a person had become known to police after the previous checks, the agency may not be aware of this," Mr Vine said.
"The risks to staff and members of the public of such an oversight are potentially considerable."
Briefings which contained personal and sensitive information about people suspected of immigration offences were carried out in the street while others failed to note that a potential target was four months pregnant, the inspectors found.
Immigration minister Damian Green said: "Enforcement activity is the cornerstone of our new immigration system and our intelligence-led approach means we are working smarter; focusing resources where they matter the most.
"We have already launched two nationwide campaigns to close bogus colleges, tackle illegal working and reduce sham marriages.
"Using intelligence from a variety of sources, including members of the public, we carried out 1,400 arrests, 330 prosecutions and 260 removals.
"Let the message be clear, the UK is no longer an easy touch for illegal immigrants."