Midlands Metro developers have "not covered themselves in glory", it has been claimed, after it was announced trams would not run to New Street station until May at the earliest.
But now regional transport body Centro has announced it has only been given the go ahead to begin overnight tests on the track from April 23.
Birmingham’s Conservative shadow cabinet member for transport Timothy Huxtable said: "It is getting later and later all the time. I don't think anyone has covered themselves with glory on this.
"It was supposed to be opened for 'Super September', and then by the end of the year. Now, with testing only beginning at the end of April, it will be May at the earliest.
"If we can't get the delivery of these projects right, there is a danger we will open the HS2 station at Curzon Street and there will be nothing to connect to it."
The next phase of the Metro to Centenary Square, already under way, was initially designed to be running by the end of 2017 but will not now be finished until 2019.
Centro has announced it has been given the green light by Network Rail to run the overnight tests on the New Street link from April 23. This ensures it does not interfere with the operation of the rail station.
Centro's Metro programme director Phil Hewitt said: "This is a crucial piece of work because, once done, it clears the way for us to carry out the final testing, commissioning of and staff training for the new extension.
"Once that process is completed, we can start running trams down to New Street station."
Trams returned to the streets of Birmingham for the first time in more than 60 years in December when the Midland Metro started running to a new stop in Bull Street.
But work to complete the remainder of the route along Corporation Street and Stephenson Street to New Street station was suspended for eight weeks in the run up to Christmas to provide a more attractive environment for shoppers. Work restarted in the new year.
Jonathan Cheetham, chairman of Retail Birmingham Business Improvement District, welcomed the announcement.
He said: "We are delighted that this next phase has been achieved and we look forward to seeing trams arriving at Grand Central. This development is good news for the city and traders."
The New Street extension was part of a £128 million package of upgrades to the Metro along with the new depot at Wednesday and a fresh fleet of trams.