The UK Border Agency has cut too many staff too quickly and is now having to hire extra people, Whitehall's spending watchdog has said.
The National Audit Office (NAO) said instead of slowing down staff cuts when it emerged an automated system designed to save money was both a year late and tens of millions of pounds over budget, the UKBA increased the speed of its planned changes.
More than 1,000 jobs over and above the planned reductions were cut last year, performance dropped and there is little evidence of the strong leadership needed to resolve the problems, the report added.
Border Force staff working at Heathrow Airport, one of its most high-profile and oft-criticised operations, also appeared reluctant to take up more changes.
Changes were also brought in cautiously, "partly through concern about industrial relations, but also piecemeal, without evaluating their potential impact", the report said.
A lack of integration, with some 120 separate targets and significant changes being made "independent of headcount reduction", affected both efficiency and performance, the watchdog said.
Some 22,580 staff were employed by the UKBA, including the Border Force, in April last year, but this had dropped to 20,469 by April this year, figures showed.
"In 2011-12, the agency's workforce reduced by over 1,000 more than planned, despite the fact that progress was slower than expected in the ICW programme and workforce modernisation at the border, and no agency-wide skills strategy was yet in place," the report said.
"The result of this disconnect was, in some places, a dip in performance and the need to hire new staff or increase overtime."
It added: "Agency performance has dipped in some specific areas, in part due to implementing staff reductions faster than originally planned.
"For example, performance in London and the South East has come under pressure due to staff shortages."
More people than expected wanted to leave the organisation, with early exit costs amounting to £60m between 2010 and 2012, the watchdog added.
The £385m immigration case work (ICW) computer system, which aimed to improve efficiency and cut costs, has "significant problems" and, despite early successes, "has slipped by a year and is over budget", the NAO said.
Despite less being delivered than expected, the system was £28m (12%) over its £224m budget by the end of March and overall expected savings had been revised down to £106m.
Shadow immigration minister Chris Bryant said: "This NAO report proves what we've been saying all along - that the Home Secretary's cuts to Border Force and Border Agency staff have gone too far, too fast and have seriously undermined their performance, with over 2,000 staff members being lost since April last year.
"As a result of this huge reduction and with cuts set to continue, relief staff are being drafted into airports such as Heathrow and Stansted, being paid overtime, being given travel and subsistence expenses and accommodation in local hotels, in some cases 4-star hotels, to shore up our borders."