The National Transportation Authority has said the longer Luas trams have been withdrawn from service on the Luas Cross City line due to a fault.
NTA chief Ann Graham said a fault had appeared in the longer trams, which means they are not available at peak times.
She said every effort was being made to identify the fault and it was expected the trams would be brought back into service within weeks resulting in a better service on the Green Line.
The longer trams have been blamed by some for traffic delays and slower travel times in Dublin city centre.
In November it was announced that eight longer trams were being added to the fleet at a cost of €89m, with the 26 existing trams also being extended by 12m.
The Cross City extension opened in early December but by the beginning of February the introduction of the 55m trams had begun to cause traffic issues.
On the second day of its operation one of the trams failed to clear O'Connell Bridge northbound with the rear of the carriages jutting out and blocking traffic on the South Quays.