By John Pullman

CALAIS, France (Reuters) - Hundreds of passengers were stranded for hours on five Eurostar high-speed trains after reports that migrants were blocking the tracks leading to the tunnel under the English Channel and were attempting to climb aboard.

According to the Eurostar Twitter feed, three of the trains then went on to London early on Wednesday, while two others returned to their original departure stations in London and Paris.

Eurostar had announced on Twitter late on Tuesday that it was holding its trains "whilst authorities deal with trespassers" and then said the trespassers had been spotted on the tracks at the station in Calais. The trains stayed in place as the trespassers were removed from the tunnel and away from the tracks, it said.

Local media reported that the trespassers were migrants attempting to cross into the United Kingdom through the Eurotunnel, also known as the Chunnel. So far this year, thousands of refugees have attempted to make the dangerous crossing.

Passengers on one of the London-bound trains, which stopped less than a mile (1.6 km) from the tunnel, were told at one point to be very quiet and listen for the sound of people climbing on the roof. A helicopter with a searchlight then circled the train as guards walked the tracks looking for migrants, but seemingly none were spotted.

With the power out, passengers on that train sat in the dark for nearly four hours. The heat and mugginess in the cars rose as conductors walked the aisles with wind-up torches. A woman in business class wept and many passengers said they could not breathe in the stifling air.

Eurostar then pulled the train back to Calais, where passengers disembarked for fresh air and bottled water. But when some attempted to leave the station to cross the English Channel by ferry, French gendarmes stopped them, saying that customs and immigration laws required that the passengers return to Paris. However, Eurostar tweeted that the train, ES9055, "is unable to be used to take passengers in Calais back to Paris and alternative options are being sought."

"We've been abandoned on a platform somewhere in the cold with NO INFORMATION," tweeted one of the passengers, Danny Bell.

"And enlighten me - at 4am in the middle of nowhere what are those options? For hundreds and hundreds of people?"

Eurostar has offered to exchange tickets.

(Writing by Lisa Lambert; Editing by Peter Cooney and Eric Beech)