All tram services have been temporarily cancelled in Greater Manchester following a communications problem.
It follows a "network-wide failure" in the morning, which caused delays, a Metrolink spokeswoman said.
She said services had been suspended at 15:45 BST to allow engineers to fix the problem.
Metrolink said it "apologised for the inconvenience", adding that it was not believed to have been caused by hacking.
A spokeswoman said there had been a communications failure between the control room in the depot and trams on the network.
She added that the operator was unable to estimate when services would resume. Commuters are advised to use their tram tickets on local buses and Northern Rail services instead.
Extra staff would be available to advise at tram stations, Metrolink said.
In May, the tram network was shut down because control room staff lost visible sight of trams.
Metrolink tweeted that it planned for "services to run as usual" from Sunday morning "following disruption to the network on Saturday".