The Government alone is powerless to close a loophole which enables people to enter the UK illegally, the Immigration Minister admitted today.
Damian Green said the so-called "Lille loophole", in which passengers miss key border checks by buying a train ticket from Brussels to Lille but then stay on until London, could only be closed through negotiations with European countries.
People can travel freely between Brussels and Lille as both Belgium and France are in the Schengen agreement, which allows them to cross the borders without passport checks.
But the UK Border Agency (UKBA) operates border controls in the country of departure, so by staying on the train from Lille, no passport checks are performed on the route into London, BBC Radio 4's The Report found.
Mr Green told the programme: "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."
Jonathan Sedgwick, the former UKBA acting chief executive who is now the agency's international group director, is expected to be questioned over the loophole when he appears before MPs on the Commons Home Affairs Select Committee today.
It comes after senior officials at the UK's border force scrapped key checks against a Home Office database without ministerial approval, allowing an unknown number of suspected terrorists, criminals and illegal immigrants into the UK.
Border force staff have also been threatened with arrest by the Belgian police for trying to close the loophole, emails seen by The Report showed.
The programme said one officer, who tried to question two people suspected of trying to take advantage of the loophole, was told by Belgian police: "This has got to stop, you are not in Britain now, you are in Schengen. If they make a complaint, you will be arrested."
Another UKBA officer warned that staff were so scared of being arrested by the Belgian police that they "will now turn a blind eye to potential Lille loopholers".
:: BBC Radio 4's The Report can be heard at 8pm tonight.