Transport for Greater Manchester's £1.4bn expansion of the light rail system around the city region has been delayed due to problems arising from introducing the new computerised management system.
The lines to Central Park, the business park in east Manchester, and Oldham Mumps were due to open in December 2011 and March 2012 respectively. They will now open at the same time in 2012 but the date has not been given.
TfGM said the system was undergoing testing by contractor Thales and an update on the launch dates would be given in January.
The expansion is tripling the size of the network as it is now with a line to Rochdale via Central Park and Oldham, a line to Ashton-under-Lyne, a line to East Didsbury and another to Manchester Airport via Wythenshawe, plus a second crossing through Manchester city centre. The new South Manchester Line opened in July this year.
Philip Purdy, TfGM's Metrolink director, said: "The new operating system will provide benefits for existing passengers, not least real-time service information, but it is also a fundamental part of the expansion: ultimately, we simply cannot have one without the other.
"That is because the existing signalling on the Altrincham and Bury lines would not be able to support the number of tram movements that the expanded network will bring on to the network, particularly at a number of pinch-points on the network.
"The first stage of introducing the new system is to integrate the new with the old in a way that has the least impact on existing services.
"We are working closely with the contractor, Thales, on this and the progress made ultimately dictates when lines can open for service.
"We are currently assessing the implications this has for when we can start running to Central Park and Oldham Mumps, which are the next sections in line to open, but we will open them together, rather than in two phases, next year. An update will be presented to TfGMC in January.
"We share the frustrations of people waiting patiently to use the services, especially as construction of the new lines is going well. However, this is about the safe operation of a network that will be three times the size it is now, so we cannot cut any corners – we have to be thorough."
Cllr Andrew Fender, chairman of the TfGM Committee, said: "This is a very complex, technical issue that officers are working closely with the contractor to address.
"I fully appreciate that people want to be able to use their new Metrolink services sooner rather than later, but ultimately this is about ensuring the safe and reliable operation of the expanded network. So it cannot be rushed; we have to get it right."