British border employees have faced threats of arrest by Belgian police for attempting to stop people exploiting a loophole permitting them to enter the UK without passport checks, the BBC has discovered. Passengers can journey from Brussels to Lille without showing passports as Belgium and France are each within the Schengen area, external. But they will then stay on the practice to the UK as there aren't any further checks. The house Office confirmed it was working to deal with the "Lille loophole". The Schengen agreement, exterior allows passport-free journey between some European countries, but the UK shouldn't be a signatory. Passengers with tickets from Brussels to Lille can board the train to London with out showing a passport, but no-one checks whether passengers truly get off the practice at Lille and UK Border employees do not carry out checks on board. So passengers can continue on to London St Pancras, with out going by means of further passport control, and illegally enter the nation. Radio 4's The Report, exterior has seen inner UK Border Agency, exterior (UKBA) correspondence highlighting the loophole, and the threats being made to workers trying to tackle it. One UKBA officer describes an incident in April at a Brussels station where he stopped two Iranians who he stated "bore all the hallmarks of Lille loopholers". After they have been questioned the Belgian police intervened. The BBC has seen other emails describing clashes this year between UK Border Agency workers and Belgian federal police. This led one officer to warn that many employees he had spoken to had been so frightened of being arrested by the Belgian police that they "will now turn a blind eye to potential Lille loopholers". The problem has been mentioned at the very best levels of the UK Border Agency and the Foreign and Commonwealth Office, exterior. An e-mail from a senior director to staff in May this year acknowledged that the British Ambassador to Belgium had "voiced UKBA concerns on border controls for Lille passengers" to the Belgian immigration minister. The home Office, exterior minister Damian Green confirmed that both he and the home Secretary Theresa May are conscious of the loophole.