The four men - all believed to be Russians - managed to sneak past the usually tight security in the Eurostar terminal at the Gare du Nord in Paris, entered the train via an inspection hatch and hid in a cramped compartment beneath the floor of the train.
Along flat stretches of the track between Paris and Calais, the train reached a top speed of 186mph, a spokesman for the rail company said.
The train left Paris at 7.10am and travelled 240 miles - including passing through the tunnel - to Ashford in Kent before the men were discovered by Eurostar staff who heard tapping coming from beneath the floor when the train stopped there at 9.10am. They were arrested by British Transport Police and were being questioned last night by immigration officers at Dover.
"They were not actually hanging on under the train as they had [another] floor beneath them. But it would have been very cramped indeed," said a Eurostar spokesman in London.
"We've never had anything like this before and we've started an investigation as to how they managed to get through security at Paris," he said.
The compartment the men hid in measured between 18in and 2ft deep, and 6ft wide, with a thin metal floor. The space is reached via an inspection hatch on the outside of the train which the men must have used while backs were turned in Paris.
Police said they were still trying to confirm the identity of the four men and where they came from.
It is not the first time stowaways have made it to Britain via the Channel Tunnel. A man from the former Yugoslavia sneaked in on a tunnel freight train in June 1995 by hiding under a car. He was caught at Folkestone.
But yesterday's incredible journey is thought to be the first example of anyone stowing away on a Eurostar train.Reuse content