Illegal immigrants are able to climb aboard British-bound lorries in Calais despite supposed fool-proof searches, an influential peers' group has warned.
The Home Office has spent millions on scanning equipment at Calais
A House of Lords committee discovered inadequate fencing at the French port would allow illegals to hide in parked vehicles after security checks.
The peers estimated that 1,500 migrants a year could be stopped from reaching the UK if the fences were replaced.
The Tories said the news exposed how porous UK borders were under Labour.
The peers' discovery, made during a visit to Calais as part of an investigation into Europe's borders, comes despite the Home Office spending millions on high-tech equipment in Calais and other EU ports.
This has included: heartbeat sensors, CO2 probes to detect exhaled breath and "passive millimetre wave" scanners which can "see" through vehicles.
The equipment has been in Calais - through which more than 800,000 vehicles pass each year - since October, 2004.
Lord Jopling, chairman of the House of Lords EU Committee on Home Affairs, said: "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.
"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.
"This would be entirely in British interests. The British and French authorities must take immediate action to remedy this."
The peers said the poor fencing was the "one weak point in an otherwise excellent system".
But shadow home secretary David Davis said: "This revelation exposes yet again how porous our borders have become under Labour.
"This, despite 10 years in power and millions spent on technology trying to tackle the problem.
"The government must take action with the French authorities to ensure they meet their responsibilities in Calais.
"And Gordon Brown must answer our calls for a dedicated border police force - with proper police powers - to protect the UK from the scourge of illegal immigration and the crime that flows in with it."
Liberal Democrat home affairs spokesman Chris Huhne said the report "highlights once again the difference between the Government's tough talk on immigration and its lack of action".
"Rhetoric is not going to secure our borders, but a national border force and proper entry and exit controls will."
A Border and Immigration Agency spokesman insisted that "maintaining the security and integrity of our borders is a key priority".
"Our borders are some of the toughest in the world," he said.
"Last year we searched over one million lorries and prevented a record 18,000 attempts by illegal immigrants to cross the channel.
"But we are not complacent and must continue to respond to the challenges that mass migration brings."
Schengen decision backed
The spokesman added: "To further tighten controls we are introducing biometric IDs for non-European Economic Area foreign nationals from 2008 and have doubled resources for enforcement.
"This is backed by new powers for immigration officers at our borders as part of the UK Borders Act and removing incentives for illegal immigrants coming to Britain."
The Lords committee also supported the government's decision not to join the European Union's Schengen agreement, which allows passport-free travel across participating European states.