Up to 200 refugees stormed the Eurotunnel terminal near the French port of Calais overnight Saturday, forcing the suspension of rail services for several hours.
About half of the group encroached on railway tracks after breaking through fences, the BBC reported.
The company said the breach was "clearly an organized attack" after it tightened security several times in recent weeks.
Services resumed on Saturday morning, with waits of about 30 minutes, after they were suspended for about eight hours, it said.
Eurostar, which operates trains to Britain through the tunnels under the English Channel, said later Saturday morning that some of its trains were held outside one of the two tunnels pending safety checks.
"Trains are currently sharing one tunnel while safety checks are carried out," Eurostar said on Twitter.
Dozens of minor breaches of security cordons are reported nightly, Eurotunnel has said.
The company and French police tightened security in July and August after the number of refugees trying to force their way towards the tunnel had risen to some 2,000 nightly in June.
Eurotunnel said it had intercepted 37,000 refugees this year by the end of July, while more than a dozen people have died during recent attempts to find a way through the tunnel.
Many refugees live for months in crude encampments around Calais as they await a chance to reach Britain.
By Bill Smith
