There was a "critical situation" at the freight yard

Downing Street said there was a problem last week when the unit was reassigned, but said press coverage had been "exaggerated".

A spokesman said: "The French have told us that a full replacement squadron of 50 officers have now arrived there.

"In addition SNCF (the French rail operators) are to begin installing better physical security fences and surveillance equipment next month."

Eric Martos, spokesman for French rail operator SNCF which runs the depot, said the yard had been left short of security from Friday until the squad was returned on Tuesday.

He said the local gendarmerie remained in place all weekend, but proved overwhelmed by sheer numbers.

"On Friday about 300 asylum seekers attacked the yard and it was impossible to stop them and it was a very critical situation," he said.

British rail-freight operator EWS said the change in policing allowed a wave of illegal immigrants through the tunnel.

Security 'inadequate'

It said it had found 26 asylum seekers trying to enter Britain illegally at its freight yard in Folkestone on Sunday, and a further 45 on Monday.

EWS has long called on the UK government to do more to press the French authorities on security at the freight depot.

Illegal immigrants caught in Folkestone

March: 111 April: 304 May: 202 so far Source: British Transport Police

It said restrictions on its services, caused by asylum seekers jumping the trains, have cost it �10m in the last six months.

A Home Office spokeswoman told BBC News Online security at French freight depots was "inadequate".

She said: "Security around the Channel Tunnel depots needs to be increased, but we have been told that in June, SNCF (French railways) will install hi-tech security measures."

EWS welcomed the new measures, but said France had a long way to go to guaranteeing a normal service for its trains.

EWS spokesman Graham Meiklejohn said: "The British Government needs to step in and act decisively once and for all."

British Transport Police said 340 illegal immigrants crossed from Frethun to ESW's freight yard in Folkestone in April, up from 111 in March, and that 202 asylum seekers had already been discovered in Folkestone so far this month.

"There has clearly been an increase," Police Sergeant Danny Porte told the Reuters news agency.