Passengers can travel freely from Brussels to Lille without showing passports, as Belgium and France are both within the Schengen area.

But some then stay on the train, continuing to London St Pancras without being challenged as there are no further passport checks, due to a so called Schengen agreement which allows passport-free travel between some European countries.

However, the UK is not a signatory to the agreement and now the Home Office has confirmed it is working to tackle the “Lille loophole”.

The loophole was uncovered by BBC Radio 4’s The Report, which has seen correspondence among the UK Border Agency (UKBA) highlighting threats being made to staff trying to tackle the loophole.

One UKBA officer describes an incident in April at a Brussels station where he stopped two Iranians who he said “bore all the hallmarks of Lille loopholers”.

After they were questioned the Belgian police intervened.

One officer shouted: “This has got to stop. You are not in Britain now, you are in Schengen. If they make a complaint you will be arrested”.

The BBC has seen other emails describing clashes this year between UK Border Agency staff and Belgian Federal police.

This led one officer to warn that many staff he had spoken to were so scared of being arrested by the Belgian police that they”will now turn a blind eye to potential Lille loopholers”.

The Home Office minister Damian Green confirmed that both he and the Home Secretary Theresa May are aware of the loophole.

“Any loophole in our border defences is a matter of concern,” he told The Report.

“It’s one of those things that the British government can’t solve on its own… that has to be solved in negotiations, and that’s what we’re now doing.”

He added: “There are strict British immigration controls in place in France and Belgium and we have UK Border Agency officers based at St Pancras to target those we believe are intent on entering Britain illegally.

“We are currently working closely with our Belgian counterparts and Eurostar to resolve this as quickly as possible.”

The chairman of the Home Affairs select committee Keith Vaz said, “If a simple railway ticket can give you entry into the UK, this is a cause for real concern.”