Coach passengers were allowed into the UK without being properly checked by border staff for four years, it has been reported, in another blow for the under-fire Home Office.
A relaxation quietly introduced under the last Labour administration to ease queues at the port of Dover in 2007 was only halted 10 days ago when senior border officials were suspended, the Sunday Telegraph said.
Officials checked only if passport photographs matched the bearers, the newspaper said, and did not cross-check against computer databases of suspected terrorists, criminals and immigration offenders.
The extent of the watering down of security, such as whether it applied to other ports and was only employed at peak periods, remains unclear but the number of arrivals involved was estimated to be in the millions.
A Home Office spokesman said he was unable to discuss the claims because of the ongoing inquiry into unauthorised relaxations of controls, in what appeared to be an indication that they formed part of the probe.
But MPs will seek more information on Tuesday when the Home Affairs Committee questions Brodie Clark, the former head of the UK Border Force who quit his post amid an acrimonious dispute with Home Secretary Theresa May.
Mr Clark was suspended last week by UK Border Agency chief Rob Whiteman, who says he admitted allowing border staff to relax checks beyond the extent of a pilot scheme authorised by Mrs May.
He denies exceeding his authority and has left his post to pursue a claim of constructive dismissal.
A Home Office spokesman said: "Nothing is more important than the integrity of our border in order to protect national security and reduce and control immigration. There are ongoing investigations into allegations regarding the relaxation of border controls without ministerial approval."
Security checks at Heathrow airport were also watered down during a two-day strike by immigration officers in October last year, it was reported. The Home Office said: "UKBA ensured additional trained staff authorised to carry out all necessary checks were in place at the border during the strike last year."