The Home Office has announced immediate action to close a loophole which allows passengers to travel to Britain on the Eurostar without any form of identification.
Journalists were able to travel between Brussels and London without a single check, the Daily Mail reported.
A Home Office spokesman said: “We are committed to ensuring that all necessary measures are taken to protect the security of our border.”
“It is the responsibility of Eurostar to ensure that all UK bound passengers are presented to UK authorities for examination at the border.
“It would appear the operator has not adhered to the pre-agreed, robust processes we have in place in this instance.
“As a result, we have taken immediate action to introduce additional checks in a number of locations, including Brussels, Lille and St Pancras stations, while this issue is resolved.”
A Eurostar source said the pre-agreed process of dealing with passengers was adhered to.
Britain is not a member of the Schengen Zone, which allows passport-free travel across most of the EU’s member states.
Under a loophole, passengers can board the Eurostar without identification in Brussels with a ticket to the French city of Lille, and then remain on the train when it heads to London, according to the Mail’s investigation.
A £240 monthly “abonnement” commuter ticket for frequent travel between Brussels and Lille could be obtained without having to supply a passport or driving licence, the paper reported.
Lille-bound passengers generally travel in a specific coach controlled by security guards who ensure they all get off in France.
But a different approach is adopted on a rush-hour early evening train and it was claimed that when a reporter pulled into Lille on this service no checks were made and he remained on the train bound for London.
A Eurostar source insisted that the pre-agreed process of dealing with passengers travelling on the 5.56pm Brussels train was adhered to.
The company said in a statement: “Immigration requirements are defined and controlled by the relevant authorities in each country that we operate in, and we work very closely with these authorities to ensure that our services operate within their legislation.
“For passengers travelling between Brussels and Lille, the authorities have put a special arrangement in place.
“This allows Brussels-Lille passengers to travel in a separate coach controlled by security guards, who then ensure that all of these passengers disembark at Lille before the train goes on to the UK.
“For the 17.56 train, which is the most popular commuter service, the border authorities have chosen to operate a slightly different process whereby the train is only bookable by season ticket holders who do not have to travel in a separate coach.
“This was agreed because they are a defined group with ID and contact details checked when buying their season ticket.
“In addition, the UK border authorities regularly undertake random immigration checks on arrival at London St Pancras.”