Tram chiefs said the vehicles could not cope with the icy weather

"Unusual" weather conditions overnight were blamed for the delays, which caused chaos for thousands of people.

Ice on the overhead lines and frozen points was not removed by a special "ice-breaking" tram which was sent out by operators Metrolink.

It said the ice had reformed on the tracks after the vehicle had broken it up.

A spokeswoman said an investigation had begun into how similar problems could be avoided in the future.

She added: "We had a very unusual night in that we had a freeze, followed by a thaw, followed by a freeze again.

"The ice breaker vehicle was going out and then having to go out again the clear up the ice.

"There were delays of an hour-and-a-half and we advised passengers to use alternative transport."