The UK Border Agency (UKBA) is to be broken up after further revelations of relaxations of checks on immigrants to Britain, Theresa May has told the Commons on Monday afternoon.

The UK Border Force - which physically runs the checks on travellers at airports and Eurostar terminals - will be split from the UKBA, because Theresa May believes managing all the operations is "too great for one organisation."

The decision, which comes into effect on the 1st of March, is likely to cause a significant shakeup to the policing of Britain's borders. Theresa May told MPs: "Many of the changes I've outlined cannot happen overnight. They will take time, but we will make them as quickly as possible."

The decision comes in the wake of a report by John Vine, the independent inspector of the UK Border Agency, who was called in to investigate the lapses in passport checks which led to the resignation of Brodie Clark. The former head of the UK Border Force was accused of exceeding the terms of a pilot scheme over the summer, which reduced some checks on passengers.

John Vine's report does not touch on the involvement of Clark in the passport checks fiasco - Vine's remit was to look at the relationship between the UK Border force, its managers and ministers. His report reveals a catalogue of failed communications between the UK Border Force, the UK Border Agency, and ministers in the Home Office.

The home secretary revealed that the relaxation of checks at the border - carried out without the government being made aware - was far more widespread than previously thought. Lapses and inconsistencies had been happening as far back as 2007. Theresa May said that some of the relaxations, particularly on checking Eurostar passengers against the Warnings Index of potential terror suspects, had been low-risk.

But she said that other suspensions had been "completely unauthorised and that is simply not acceptable."

Checks against the Home Office Warnings Index were not carried out on about 500,000 European Economic Area (EEA) nationals travelling to the UK on Eurostar services from France. This led to hundreds of thousands of people not being checked against the list of terror suspects.

Theresa May said some relaxations of Warnings Index checks had been authorised by the previous Labour government "quite reasonably". But John Vine's report, published on Monday, reveals that many other checks were abandoned by UK Border Force staff, without the permission or knowledge of ministers.

However she wouldn't be drawn on whether her pilot scheme last summer - intended to lead to what the Home Office says is a more "intelligence-led" approach to border policing - had been a success. Because UK Border Force staff had been exceeding the terms of the pilot, the Home Secretary said it was not possible to give an indication about what effect the pilot had.

"We keep an open mind," she said.

Given the furore surrounding the pilot scheme when the Brodie Clark affair surfaced last autumn, a repeat of it seems highly unlikely.

Shadow home secretary Yvette Cooper - who had been a Treasury minister in the last government at the time problems at the UK Border Agency were emerging - pointed out that the number of times checks had been abandoned at Heathrow had risen steadily since the coalition had been formed.

In the first nine months of 2011 they were up 51% on the previous year. Cooper suggested that staffing cuts at the UKBA were to blame.

Cooper accused ministers of not doing proper monitoring of the pilot they introduced, saying they'd taken "a huge experiment with border security."

Responding Theresa May accused Cooper of "mock outrage", saying that Labour had gone into the last election with a clear intention of cutting the UK Border Agency's budget.