Illegal immigrants are bypassing supposedly foolproof border controls by simply climbing over a fence at Calais.
The farcical discovery was made by Peers during a visit to the French port - where migrants assemble to try to sneak into the UK by lorry.
The House of Lords committee said inadequate fencing at Calais was allowing "determined" illegals to get into a "clean" lorry park - where vehicles have already undegone checks involving heartbeat and breath sensors and scanners which can see through vehicles.
Last year 1,300 were found in the parking area, but critics say thousands more are likely to have made it to the UK.
More than 800,000 vehicles pass through Calais each year.
It is hugely embarrassing for the Home Office, which has spent millions providing hi-tech scanning equipment in Calais and other European ports.
The heartbeat sensors, CO2 probes and "passive millimetre wave" scanners which can see through vehicles have all been provided at the UK taxpayer's expense. The equipment has been operating at Calais since October 2004.
Once these checks have been cleared, lorries park near the ferry berths and the House of Lords EU Committee on Home Affairs noticed the inadequate fencing.
The chairman of the committee, Lord Jopling, 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 report said the poor fencing was the "one weak point in an otherwise excellent system".
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."
Sir Andrew Green, chairman of Migrationwatch UK, added: "It's amazing that it takes a visit by a group of Lords to discover a blatant gap in our border defences.
"Despite the spin about tough border controls we now find we have been wasting our time and money."
The report will spark particular alarm at a time when the mayor of Calais hopes to build a new refugee centre, dubbed "Sangatte II".
The original Sangatte camp, near Calais, attracted thousands of refugees, who regularly made the short walk to the nearby Channel Tunnel entrance to try to jump on to slow-moving trains.
The Peers backed the Government's decision not to join the European Union's Schengen agreement, which would force the UK to drop its border controls for EU travellers.
A Border and Immigration Agency spokesman said last night: "Our border with France is one of the toughest in the world. Last year we searched over one million lorries and prevented a record 18,000 attempts by illegal immigrants to cross the channel.
"We work very closely with our French counterparts to ensure that port security is tight.
"The fence at Calais port is alarmed and French authorities maintain regular patrols in this area. CCTV cameras linked to the resident French police office monitor the area at all times.
"BIA also employs private contractors to patrol the berthside area and intercept those who breach the security fence."
Observers in Calais said the "flimsy" fences were only 6ft tall. Very few have barbed wire running along the top, and none are electrified.
Once through security, the lorries often have to queue for hours - often with their engines running - before being guided on to the ferries.
A spokesman for Calais port confirmed the fences were "regularly attacked", and that patrols had to work "round the clock to repair them".
He added: "It is a never ending job. Once we rebuild a fence, the illegal immigrants try to cut through them, or tear them down again."
