Nottingham's £570m tram extension, which has been hit by a series of delays, may not open until April, according to the city council.
The authority said it had been told by contractor Taylor Woodrow Alstom the extensions to Chilwell and Clifton were facing lengthy delays.
But Vinci - Taylor Woodrow's French parent company - described the comments as "pure speculation".
An industry expert said the firm could face substantial penalties over delays.
Deputy leader of the city council Graham Chapman said the project may not be finished until Easter at the earliest, rather than the last-given finish date of December.
"We're not very happy about it. The contractors are ultimately responsible for it but it could take some time," he said.
When it began in 2011, the project was due to finish by the end of 2014.
But problems with redirecting utilities pushed the end date back. Another delay was blamed on problems minimising noise and vibrations along the route.
Aaron Morby, editor of industry magazine the Construction Enquirer, said Vinci would face extra charges including increased staffing costs, dealing with problems the project has faced and "liquidated damages penalties" - fines payable if a project is delayed.
"Put together and assuming the project is delivered three to four months late this could run into tens of millions of pounds," he said.
"It focuses the mind about whether or not the council would be keen to pursue further extensions to the project. Certainly UK-wide it does raise questions about tram links themselves."
Investor relations director at Vinci Construction, Christopher Welton, was quoted in the Construction Enquirer saying the tram extension had caused it to "scale back activity" elsewhere in Europe.
"Our new management team is in place and concentrating on getting the problem project out of the way before they turn to developing new business," he said.
"We don't know what the exact timing of that will be."
In a separate statement to the BBC, Vinci said: "We are aware of reports suggesting a specific timescale for the launch of the Phase Two network but these remain pure speculation.
"Construction work is nearing completion and the contractors are working to fully undertake the testing and commissioning phase as soon as possible. Some months ago we said the launch would take place during the first part of 2015 and this remains the case."