Illegal immigrants are able to bypass hi-tech security and sneak onto lorries waiting to cross to Britain because of faulty fencing in Calais, a House of Lords committee said on Wednesday.
The peers spotted the breach on a visit to the French port as part of an inquiry into European border security.
They were told that 1,500 migrants were found every year hiding on lorries in a car park near ferry berths.
Before reaching the car park, the lorries are searched with sniffer dogs, carbon dioxide probes, heartbeat sensors and radiation imagers, and thus assumed to be free of stowaways.
But the peers said the car park was too large and the fencing inadequate, allowing illegal migrants to get through.
"This seems to us to be the one weak point in an otherwise excellent system," said the House of Lords EU Home Affairs Committee in a report.
It said more effective fencing should be put in place as a matter of urgency.
"Our visit to the juxtaposed British and French border controls at Calais showed that there are excellent new technologies deployed to prevent illegal migration to the UK," said the committee's chair, Lord Jopling.
"However we were surprised to find inadequate fencing, the replacement of which might prevent 1,500 illegal immigrants a year from boarding lorries bound for Britain.
"The British and French authorities must take immediate action to remedy this."
The UK Border and Immigration Authority stopped almost 17,000 people from crossing the Channel to Britain illegally in trucks in 2006, and refused entry to a further 6,800, the report said.
